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SUMMARY TYPE I TYPE II TYPE III TYPE IV TYPE IV

Immunology · Master Comparison

Hypersensitivity Types I–IV

One high-yield comparison of the mediators, targets, mechanisms, timing, examples and diagnostic clues for the four accepted Gell and Coombs hypersensitivity categories.

Feature	Type I Immediate	Type II Antibody-mediated	Type III Immune complex-mediated	Type IV T-cell–mediated
Principal mediator	IgE	IgG or IgM	Predominantly IgG; IgM may participate	CD4⁺ and CD8⁺ T cells
Antigen or target	Soluble environmental allergen	Fixed antigen on a cell surface, receptor or extracellular matrix	Soluble antigen forming circulating immune complexes	Soluble or cell-associated antigen presented to T cells through MHC molecules
Core mechanism	Allergen cross-links IgE bound to FcεRI on mast cells → degranulation	Antibody binds a fixed target → cell removal, inflammation or altered receptor function	Immune complexes deposit in tissues → complement activation and neutrophilic inflammation	Sensitized T cells produce cytokine-mediated inflammation or directly induce target-cell apoptosis
Major effector pathways	Mast-cell and basophil degranulation Late eosinophilic inflammation	Complement-mediated lysis Opsonisation and phagocytosis ADCC by NK cells Receptor stimulation or blockade	Complement activation Neutrophil recruitment Frustrated phagocytosis Vasculitis and fibrinoid necrosis	TH1–macrophage activation TH17–neutrophil recruitment CD8 cytotoxicity Granuloma formation
Key mediators	Histamine Tryptase Leukotrienes Prostaglandins IL-4 IL-5	C3b C5a MAC Fc receptors NK cells	C3a C5a Neutrophils Proteases ROS	IFN-γ TNF IL-17 Perforin Granzymes Fas ligand
Typical timing	Seconds to minutes Late phase commonly develops after several hours	Usually hours to days, depending on the target and mechanism	Arthus reaction: approximately 4–12 hours Serum sickness: commonly 1–2 weeks	Classical reaction peaks at 48–72 hours Drug reactions may take days or weeks
Classic examples	Anaphylaxis Allergic rhinitis Atopic asthma Urticaria and angioedema Food allergy	Haemolytic transfusion reaction Haemolytic disease of the newborn Autoimmune haemolytic anaemia Immune thrombocytopenia Anti-GBM disease Graves disease Myasthenia gravis Pemphigus and pemphigoid	Arthus reaction Serum sickness Systemic lupus erythematosus Post-streptococcal glomerulonephritis IgA vasculitis Hypersensitivity pneumonitis	Tuberculin reaction Allergic contact dermatitis Type 1 diabetes mellitus Multiple sclerosis Type 1 lepra reaction Acute cellular graft rejection SJS/TEN and DRESS
Diagnostic clue	Skin-prick testing Allergen-specific IgE Raised mast-cell tryptase after anaphylaxis	Direct or indirect Coombs test Linear staining in anti-GBM disease Fish-net pattern in pemphigus Linear basement-membrane staining in pemphigoid	Granular or “lumpy-bumpy” immunofluorescence Reduced complement during active disease Immune-complex vasculitis	Patch testing Tuberculin-test induration Lymphocytic or granulomatous inflammation
Dental relevance	Latex-protein allergy Chlorhexidine anaphylaxis Immediate antibiotic allergy	Pemphigus vulgaris Mucous membrane pemphigoid Immune thrombocytopenia	SLE-associated oral lesions Systemic vasculitis Renal involvement affecting prescribing	Contact allergy to dental materials Rubber-accelerator dermatitis Oral lichenoid reactions Severe delayed drug reactions
Major exam trap	First exposure usually causes sensitization; the clinical reaction usually follows re-exposure.	Type II is not always cytotoxic. Antibodies may stimulate or block receptors. ADCC is mediated mainly by NK cells, not CD8 T cells.	Type II targets a fixed antigen; Type III begins with complexes formed against soluble antigen.	Type IV is antibody-independent. Patch testing assesses Type IV; skin-prick testing assesses Type I.

“Type V” is not a separate accepted category. Receptor-stimulating or receptor-blocking antibody disorders, including Graves disease and myasthenia gravis, are classified within Type II hypersensitivity.

Four-Line Recall

Type I IgE, mast cells and immediate allergy.

Type II IgG or IgM against a fixed cellular or tissue target.

Type III Soluble immune complexes, complement and neutrophils.

Type IV T cells, delayed inflammation and direct cytotoxicity.

Immunology · High-Yield Notes

Type I Hypersensitivity

An immediate, IgE-mediated reaction produced when an allergen cross-links IgE already bound to mast cells in a previously sensitized individual.

Type I hypersensitivity is commonly associated with allergy, atopy and anaphylaxis. The immediate reaction usually develops within seconds to minutes, although a later inflammatory phase may develop several hours afterward.

Principal antibody IgE

Principal effector cell Mast cell

Dominant T-cell response Type 2 / TH2

Typical onset Seconds to minutes

Core Immunological Sequence

First allergen exposure Type 2 immune response IL-4 and IL-13 IgE production IgE binds FcεRI Re-exposure and mast-cell activation

Stage of Sensitization or Priming

Sensitization occurs during the initial exposure to an allergen. This stage is often clinically silent.

The allergen enters the body and is captured by an antigen-presenting cell, particularly a dendritic cell.
The allergen is processed and presented to naïve CD4⁺ T cells, promoting a type 2 or TH2-pattern immune response.
IL-4 and IL-13 stimulate B-cell class switching and the production of allergen-specific IgE.
IL-5 promotes eosinophil production, activation and survival.
Secreted IgE binds with high affinity to FcεRI receptors on mast cells and basophils. The individual is now sensitized.

Critical distinction: The first exposure produces sensitization. The characteristic clinical reaction usually appears after a subsequent exposure to the same allergen.

Subsequent Exposure and Mast-Cell Activation

On re-exposure, the allergen binds and cross-links adjacent IgE molecules attached to FcεRI receptors. This activates the mast cell and initiates mediator release.

Immediate Phase

Within minutes

Rapid mast-cell degranulation releases preformed mediators. Additional lipid mediators are synthesized soon afterward.

Histamine: vasodilatation, increased vascular permeability, pruritus, mucus secretion and bronchoconstriction.

Tryptase and other proteases: tissue effects and evidence of mast-cell activation.

Leukotrienes C₄, D₄ and E₄: potent and sustained bronchoconstriction with increased vascular permeability.

Prostaglandin D₂ and platelet-activating factor: bronchoconstriction and amplification of inflammation.

Late-Phase Reaction

Usually 4–8 hours later

Cytokines and chemotactic mediators recruit inflammatory cells to the affected tissue. The reaction may persist for many hours.

Important cells: eosinophils, basophils, type 2 T cells and neutrophils.

Important cytokines: IL-4, IL-5, IL-13, TNF and other inflammatory mediators.

Clinical consequence: persistent oedema, mucus production, bronchial hyperresponsiveness and tissue inflammation.

Eosinophils: release granule proteins and lipid mediators that can contribute to tissue injury.

Clinical Manifestations and Examples

Localized Type I Hypersensitivity

Allergic rhinitis and hay fever
Allergic conjunctivitis
Atopic or allergic asthma
Urticaria
Angioedema
Food allergy
Atopic dermatitis, in which IgE-mediated mechanisms may coexist with other immune mechanisms

Systemic Type I Hypersensitivity

Anaphylaxis is an acute, potentially life-threatening systemic reaction involving one or more of the following:

Airway oedema or upper-airway obstruction
Bronchospasm and respiratory distress
Generalized urticaria or angioedema
Hypotension, circulatory collapse or shock
Gastrointestinal cramping, vomiting or diarrhoea

Common triggers include foods, medicines, insect venom, latex and chlorhexidine.

Atopy refers to a genetic tendency to develop exaggerated allergen-specific IgE responses to otherwise harmless environmental antigens.

Diagnosis

Clinical history: identify the suspected allergen, timing of exposure, reproducibility and organ systems involved.
Skin-prick testing: demonstrates immediate cutaneous reactivity to a suspected allergen.
Serum allergen-specific IgE: demonstrates sensitization but does not, by itself, prove clinically significant allergy.
Serum mast-cell tryptase: may support the diagnosis of acute mast-cell activation when sampled appropriately after suspected anaphylaxis.
Challenge testing: may be used under specialist supervision when the diagnosis remains uncertain.

A positive skin test or specific-IgE result indicates sensitization. It must be interpreted alongside the clinical history before diagnosing allergy.

Dental Relevance

Potential Triggers in Dental Practice

Chlorhexidine-containing products
Latex gloves, rubber dam and other latex devices
Antibiotics, particularly beta-lactam antibiotics
Analgesics and other prescribed medicines
Topical products, flavourings and dental materials

Important Clinical Distinctions

True IgE-mediated allergy to amide local anaesthetic agents is very uncommon.
Reactions attributed to local anaesthesia may instead reflect anxiety, vasovagal syncope, intravascular injection, toxicity or sensitivity to an additive.
Many NSAID reactions arise through non-IgE mechanisms and should not automatically be classified as classical Type I hypersensitivity.
Chlorhexidine allergy should be considered when an unexplained peri-procedural reaction occurs.

Correction of a common misconception: Routine administration of a small “test dose” of penicillin is not a reliable method of excluding anaphylaxis. Allergy assessment and formal testing should be performed through an appropriate clinical pathway when indicated.

Suspected Anaphylaxis During Dental Treatment

Stop dental treatment and call for emergency assistance.
Position the patient appropriately; avoid sudden standing or sitting in a hypotensive patient.
Give intramuscular adrenaline promptly according to the patient’s age and the applicable emergency guideline.
Provide high-concentration oxygen and monitor airway, breathing and circulation.
Repeat adrenaline when indicated by the applicable protocol.
Begin cardiopulmonary resuscitation if cardiac arrest occurs.

In adult practice, many guidelines use 500 micrograms IM adrenaline as the initial adult dose. Always follow the current local resuscitation protocol.

Examination Pearls and Common Traps

Type I hypersensitivity is principally mediated by IgE and mast cells.
IgE binds to the high-affinity FcεRI receptor on mast cells and basophils.
IL-4 and IL-13 promote B-cell class switching to IgE.
IL-5 is a major cytokine for eosinophil production, activation and survival.
Histamine is an important preformed mediator of the immediate response, but it is not the only mediator.
Leukotrienes produce particularly potent and sustained bronchoconstriction.
The late-phase reaction is characterized by cellular recruitment, especially eosinophilic inflammation.
Anaphylaxis is a clinical diagnosis; treatment should not be delayed while awaiting laboratory confirmation.
Antihistamines may improve cutaneous symptoms but do not replace adrenaline in anaphylaxis.

Rapid Recall

First exposure: allergen presentation → type 2 response → IL-4/IL-13 → IgE production → IgE binds FcεRI. Re-exposure: IgE cross-linking → mast-cell activation → immediate mediator release, followed by a cytokine-driven late inflammatory phase.

Immunology · High-Yield Notes

Type II Hypersensitivity

An antibody-mediated reaction in which IgG or IgM binds to a fixed antigen on a cell surface or within the extracellular matrix, causing cell destruction, tissue inflammation or abnormal cellular function.

Type II hypersensitivity is traditionally called cytotoxic hypersensitivity . However, the term antibody-mediated hypersensitivity is more accurate because the affected cell is not always destroyed. Antibodies may instead stimulate a receptor, block a receptor or disrupt normal cell adhesion.

Principal antibodies IgG and IgM

Antigen location Cell surface or extracellular matrix

Major effectors Complement, phagocytes and NK cells

Typical onset Hours to days

Core Immunological Sequence

IgG or IgM antibody Fixed cell or matrix antigen Complement or Fc-receptor activation Cell removal, inflammation or dysfunction

Principal Effector Mechanisms

Type II reactions can be divided into three major functional mechanisms.

Cell Destruction or Removal

Opsonisation and cytotoxicity

Antibodies bind directly to antigens on blood cells or other target cells.

IgG opsonisation: Fcγ receptors on macrophages recognise antibody-coated cells and promote phagocytosis.

Complement opsonisation: C3b deposited on the target cell promotes clearance through complement receptors.

Complement-mediated lysis: formation of the membrane attack complex may rupture susceptible target cells.

ADCC: NK cells bind the Fc portion of cell-bound IgG through CD16 and kill the antibody-coated target.

Tissue Inflammation

Complement and Fc-receptor mediated

Antibodies bind to fixed antigens in tissues or basement membranes.

C3a and C5a: increase vascular permeability and promote inflammatory cell recruitment.

C5a: is a potent chemotactic and activating factor for neutrophils.

Fc-receptor activation: neutrophils and macrophages release proteases, reactive oxygen species and inflammatory mediators.

Result: tissue injury occurs even when the target structure cannot be phagocytosed.

Cellular Dysfunction

No cell destruction required

Antibodies may alter the function of a receptor, enzyme or adhesion molecule.

Receptor blockade: antibodies prevent normal ligand–receptor interaction, as in myasthenia gravis.

Receptor stimulation: antibodies imitate the natural ligand, as in Graves disease.

Neutralisation: antibodies interfere with an essential protein, as with intrinsic-factor antibodies in pernicious anaemia.

Loss of adhesion: antibodies disrupt intercellular attachment in pemphigus vulgaris.

Important correction: Antibody-dependent cellular cytotoxicity is mediated mainly by NK cells through Fc receptors. Cytotoxic CD8⁺ T cells do not require antibody and are principally associated with Type IV hypersensitivity.

Examples: Blood Cells and Alloimmune Reactions

Acute Haemolytic Transfusion Reaction

Commonly ABO incompatibility

Pre-existing antibodies bind incompatible donor red-cell antigens and activate complement, producing rapid intravascular haemolysis.

ABO antibodies are predominantly IgM.
Features may include fever, pain, hypotension, haemoglobinuria and renal injury.

Haemolytic Disease of the Fetus and Newborn

Maternal IgG crosses the placenta

Maternal antibodies against fetal red-cell antigens, classically Rh(D), cross the placenta and destroy fetal erythrocytes.

IgM does not significantly cross the placenta.
Anti-D immunoglobulin reduces maternal Rh sensitisation.

Autoimmune Haemolytic Anaemia

Autoantibodies bind erythrocytes and promote their destruction by splenic macrophages, complement or both.

Warm disease is commonly associated with IgG.
Cold-antibody disease is commonly associated with IgM and complement.

Immune Thrombocytopenia

IgG autoantibodies commonly target platelet membrane glycoproteins, promoting Fc-receptor-mediated clearance in the spleen.

Produces thrombocytopenia and mucocutaneous bleeding.
Oral findings may include petechiae, ecchymosis and gingival bleeding.

Examples: Fixed Tissue Antigens

Anti-GBM Disease

Goodpasture disease

IgG autoantibodies target the non-collagenous domain of the α3 chain of type IV collagen in glomerular and pulmonary basement membranes.

Rapidly progressive glomerulonephritis
Pulmonary haemorrhage
Linear IgG and C3 staining along the basement membrane

Acute Rheumatic Fever

Antibodies produced against streptococcal antigens cross-react with host tissues through molecular mimicry.

It is commonly taught as a Type II reaction, although antibody-mediated and T-cell-mediated mechanisms both contribute to the disease.

Hyperacute Transplant Rejection

Preformed recipient antibodies against donor ABO or HLA antigens bind graft endothelium immediately after transplantation.

Complement activation
Endothelial injury
Thrombosis and rapid graft ischaemia

Pemphigoid Disorders

Autoantibodies bind proteins within the epithelial basement-membrane zone and promote complement-dependent inflammation.

Mucous membrane pemphigoid commonly affects oral mucosa.
Bullous pemphigoid more commonly produces cutaneous disease.
Tissue separation is subepithelial.

Examples: Antibody-Mediated Dysfunction

Receptor Blockade

Myasthenia gravis Antibodies against the acetylcholine receptor, or other neuromuscular-junction proteins, impair neuromuscular transmission.

Pernicious anaemia Antibodies against intrinsic factor interfere with vitamin B₁₂ absorption. Antibodies against gastric parietal cells may also occur.

Insulin-receptor antibody syndrome Antibodies against the insulin receptor may inhibit or abnormally stimulate receptor activity.

Receptor Stimulation

Graves disease Thyroid-stimulating antibodies bind and activate the TSH receptor, producing excessive thyroid-hormone synthesis.

The target cell is stimulated rather than destroyed. Graves disease therefore demonstrates why “antibody-mediated” is more accurate than “cytotoxic.”

Loss of Cell Adhesion

Pemphigus vulgaris IgG autoantibodies predominantly target desmoglein 3, with desmoglein 1 also involved in mucocutaneous disease.

Loss of keratinocyte cohesion
Suprabasal acantholysis
Fragile intraepithelial bullae and erosions

Historical terminology: Receptor-stimulating or receptor-blocking reactions were sometimes labelled Type V hypersensitivity. They are now generally included within Type II hypersensitivity.

Diagnostic Patterns and High-Yield Tests

Intercellular pattern

Pemphigus Vulgaris

Direct immunofluorescence demonstrates intercellular deposition of IgG, often with C3, producing a fish-net or chicken-wire pattern.

Linear basement-membrane pattern

Pemphigoid Disorders

Direct immunofluorescence commonly demonstrates linear immunoreactant deposition along the epithelial basement-membrane zone.

Linear glomerular pattern

Anti-GBM Disease

Renal immunofluorescence demonstrates smooth, linear IgG and C3 along glomerular basement membranes.

Granular pattern

Immune-Complex Disease

A granular or “lumpy-bumpy” pattern is more characteristic of deposited immune complexes and therefore suggests a Type III mechanism.

Direct Antiglobulin Test

Detects IgG or complement already bound to the patient’s red blood cells.

Used in autoimmune haemolytic anaemia, haemolytic transfusion reactions and haemolytic disease of the newborn.

Indirect Antiglobulin Test

Detects free anti-red-cell antibodies present in the patient’s serum.

Used in pre-transfusion antibody screening and antenatal screening.

Oral and Dental Relevance

Pemphigus Vulgaris

Oral lesions may precede cutaneous disease.
Bullae are fragile and rapidly rupture, leaving persistent painful erosions.
Gingiva, buccal mucosa, palate and lips may be involved.
Nikolsky sign may be positive.
Histology demonstrates suprabasal acantholysis.
Direct immunofluorescence shows intercellular IgG/C3.

Mucous Membrane Pemphigoid

May present as desquamative gingivitis, bullae or irregular oral ulceration.
Blister formation is subepithelial.
Direct immunofluorescence shows linear deposition along the basement-membrane zone.
Lesions may affect conjunctival, laryngeal or other mucosal sites.
Suspected ocular involvement requires urgent specialist assessment because scarring can threaten vision.

Immune Thrombocytopenia

Oral petechiae, purpura and spontaneous gingival bleeding may occur.
Assess the current platelet count and bleeding history before invasive treatment.
Coordinate management with the responsible medical or haematology team when significant thrombocytopenia is present.

Patients Receiving Immunosuppression

Review corticosteroids, rituximab and other immunosuppressive therapy.
Consider infection risk, delayed healing and relevant blood-test abnormalities.
Minimise mucosal trauma in patients with active autoimmune blistering disease.
Avoid placing dental appliances across actively ulcerated or fragile mucosa when possible.

Common Classification Errors

“Glomerulonephritis” is not automatically Type II. Anti-GBM glomerulonephritis is Type II because antibodies bind a fixed basement-membrane antigen. Post-streptococcal and many other forms are predominantly immune-complex mediated and classified as Type III.

“Graft rejection” is not one single hypersensitivity mechanism. Hyperacute rejection is strongly antibody and complement mediated. Acute and chronic rejection involve additional cellular and vascular mechanisms.

Type II does not require cell lysis. Graves disease stimulates a receptor, myasthenia gravis impairs receptor function and pemphigus disrupts adhesion.

Type II differs from Type III by antigen location. Type II antibodies bind fixed cellular or matrix antigens. Type III reactions involve soluble immune complexes that subsequently deposit in tissues.

Type I diabetes mellitus is not the classic insulin-receptor-antibody example. Type I diabetes is principally a T-cell-mediated autoimmune disease. Severe insulin resistance caused by antibodies against the insulin receptor is the Type II example.

Recall Mnemonic

“My Blood Group Is Rh Positive”

My Myasthenia gravis
Blood Blood-transfusion reactions
Group Goodpasture disease and Graves disease
Is Immune thrombocytopenia and insulin-receptor antibody syndrome
R Rheumatic fever
h Hyperacute graft rejection and haemolytic disease of the fetus and newborn
Positive Pernicious anaemia, pemphigus and pemphigoid

Use the mnemonic only as a recall aid. In an examination, classify the condition according to its underlying immune mechanism rather than relying exclusively on a memorised disease list.

Examination Pearls

Type II hypersensitivity is mediated principally by IgG or IgM against fixed cell-surface or extracellular-matrix antigens.
C3b promotes opsonisation, whereas C5a is a potent chemotactic and inflammatory mediator.
Antibody-coated cells are removed by macrophages carrying Fc and complement receptors.
ADCC is mediated mainly by NK cells expressing CD16, not by cytotoxic T lymphocytes.
Maternal IgG, rather than IgM, crosses the placenta in haemolytic disease of the fetus and newborn.
ABO-incompatible acute haemolytic transfusion reactions commonly involve preformed IgM and complement activation.
Graves disease is caused by antibody-mediated receptor stimulation.
Myasthenia gravis is associated with impaired neuromuscular-junction receptor function.
Pemphigus vulgaris produces an intercellular fish-net pattern, whereas pemphigoid produces a linear basement-membrane pattern.
The direct Coombs test detects antibody or complement already attached to red cells; the indirect test detects circulating antibodies in serum.

Rapid Recall

Type II = IgG or IgM against a fixed target. Antibody binding causes one of three outcomes: cell removal or lysis, complement/Fc-mediated inflammation, or abnormal cellular function. The defining distinction from Type III hypersensitivity is that the antigen is located on a cell or within tissue rather than circulating as a soluble immune complex.

Immunology · High-Yield Notes

Type III Hypersensitivity

An immune complex-mediated reaction in which soluble antigen–antibody complexes form in the circulation, deposit within tissues and activate complement, producing predominantly neutrophil-mediated inflammation and tissue injury.

Type III hypersensitivity occurs when circulating soluble immune complexes are not adequately cleared and become deposited in vessel walls, glomeruli, joints, skin, serosal surfaces or pulmonary tissues. Complement activation and Fc-receptor engagement then recruit inflammatory cells, especially neutrophils.

Principal antibodies Predominantly IgG; sometimes IgM

Antigen form Soluble antigen

Major effectors Complement and neutrophils

Classic patterns Arthus reaction and serum sickness

Core Immunological Sequence

Persistent soluble antigen Antigen–antibody complexes Inadequate clearance Tissue deposition Complement activation Neutrophilic inflammation

Pathogenesis

The reaction can be understood as three linked phases: immune-complex formation, tissue deposition and complement-dependent inflammation.

Immune-Complex Formation

Soluble antigen combines with specific antibody in the circulation. Antigens may be microbial, environmental, medicinal or derived from the patient’s own tissues.

Large complexes: usually bind complement efficiently and are rapidly removed by mononuclear phagocytes in the liver and spleen.

Intermediate-sized complexes: are less readily cleared and are particularly likely to become deposited in tissues.

Antigen excess: favours the formation of smaller soluble complexes that may persist in the circulation.

Immune-Complex Deposition

Persistent complexes become trapped in tissues, especially at sites where plasma is filtered or where blood flow produces high hydrostatic pressure and turbulence.

Renal glomeruli
Small blood vessels
Synovial joints
Skin
Serosal surfaces
Pulmonary capillaries and alveolar tissues

Inflammation and Tissue Injury

Deposited complexes activate complement and engage Fc receptors on inflammatory cells.

C3a and C5a: promote vascular permeability and inflammatory amplification.

C5a: recruits and activates neutrophils.

Neutrophils: release lysosomal enzymes, proteases and reactive oxygen species when attempting to remove deposited complexes.

Platelets and coagulation: may contribute to vascular occlusion and microthrombus formation in severe vascular injury.

Characteristic pathological outcomes: immune-complex vasculitis, fibrinoid necrosis, glomerulonephritis, arthritis and tissue ischaemia.

Why Immune Complexes Become Pathogenic

Factors Favouring Deposition

Persistence of antigen in the circulation
Formation of intermediate-sized soluble complexes
Relative antigen excess
Increased vascular permeability
High hydrostatic pressure or turbulent blood flow
Local affinity between complexes and tissue structures
Impaired complement-dependent clearance

Normal Clearance

Complement fragments bind circulating immune complexes. Erythrocyte complement receptor 1 can transport these complexes to the liver and spleen, where macrophages remove them without destroying the red cell.

Defects in complement components, complement receptors or the mononuclear phagocyte system may allow immune complexes to persist and increase the risk of deposition.

High-yield distinction: The presence of an antigen–antibody complex alone does not constitute disease. Pathology develops when complexes persist, deposit in tissues and initiate an inflammatory response.

Complement and Neutrophil-Mediated Injury

Complement Activation

C3a and C5a: act as anaphylatoxins and increase inflammatory vascular responses.

C5a: is a potent chemotactic and activating factor for neutrophils.

C3b: promotes opsonisation and clearance of immune complexes.

Complement consumption: may produce reduced circulating C3 and C4 levels in active systemic immune-complex disease.

Frustrated Phagocytosis

Neutrophils attach to deposited immune complexes through Fc and complement receptors. When the complexes are fixed within a basement membrane or vessel wall, they cannot be completely engulfed.

The neutrophil therefore releases inflammatory enzymes and reactive oxygen species into the surrounding tissue. This process is called frustrated phagocytosis.

Histological pattern: small-vessel immune-complex vasculitis may show neutrophilic infiltration, nuclear debris known as leukocytoclasia, vessel wall damage and fibrinoid necrosis.

Localized and Systemic Reactions

Arthus Reaction

Localized Type III reaction

An Arthus reaction occurs when antigen is introduced into the skin or another local tissue of a person who already has high levels of circulating IgG antibody.

Local immune complexes form within vessel walls.
Complement activation recruits neutrophils.
Oedema, erythema, haemorrhage and local tissue necrosis may develop.
The reaction commonly appears approximately 4–12 hours after antigen exposure.

Serum Sickness

Systemic Type III reaction

Serum sickness develops when a circulating antigen persists long enough for antibodies to form, producing widespread antigen–antibody complexes.

Fever
Urticarial or morbilliform rash
Arthralgia or arthritis
Lymphadenopathy
Proteinuria or glomerulonephritis
Reduced serum complement levels

Symptoms commonly arise approximately one to two weeks after a first exposure and may occur sooner after re-exposure.

Serum sickness-like reaction: This clinical syndrome resembles serum sickness but may not involve demonstrable immune-complex deposition or complement consumption. It should not automatically be treated as immunologically identical to classical serum sickness.

Important Examples

Systemic Immune-Complex Disease

Systemic lupus erythematosus Autoantibodies form complexes with nuclear antigens. Deposition may affect the kidneys, skin, joints, vessels and serosal surfaces.

Classical serum sickness Circulating foreign antigen produces widespread immune complexes, complement consumption and systemic inflammation.

Cryoglobulinaemic vasculitis Circulating cryoglobulin-containing immune complexes deposit in small vessels and may affect the skin, peripheral nerves and kidneys.

Renal and Vascular Disease

Post-streptococcal glomerulonephritis Streptococcal antigen-related immune complexes form or deposit within glomeruli and activate complement.

IgA vasculitis IgA-containing immune complexes deposit in small vessels and the renal mesangium, producing purpura, abdominal symptoms, arthralgia and renal disease.

Hepatitis B-associated polyarteritis nodosa Some cases are associated with hepatitis B antigen–antibody complexes deposited in medium-sized arteries.

Pulmonary and Other Reactions

Hypersensitivity pneumonitis Inhaled organic antigens provoke a combined immune response. Type III mechanisms are important in acute disease, while T-cell-mediated Type IV mechanisms are prominent in chronic disease.

Farmer’s lung A form of hypersensitivity pneumonitis associated with inhalation of antigens from mouldy hay.

Erythema nodosum leprosum The Type 2 lepra reaction is an immune-complex-mediated inflammatory complication and should not be confused with Type II hypersensitivity.

Rheumatoid arthritis: Immune complexes contribute to synovial inflammation, but the disease has multiple immunological mechanisms and should not be regarded as a purely Type III disorder.

Diagnostic Patterns

Immunofluorescence

Granular Deposition

Immune-complex deposition generally produces a granular or “lumpy-bumpy” pattern on immunofluorescence.

This contrasts with the smooth linear antibody deposition seen in anti-basement-membrane Type II disease.

Complement

Reduced C3 and C4

Active systemic immune-complex disease may consume complement, producing reduced C3, C4 or total haemolytic complement levels.

The precise pattern depends on the disease and complement pathway involved.

Renal Assessment

Urinalysis and Renal Function

Haematuria, proteinuria, red-cell casts and reduced renal function may indicate immune-complex glomerular injury.

Inflammation

ESR, CRP and Tissue Biopsy

Inflammatory markers may be elevated. Tissue biopsy can demonstrate vasculitis, glomerulonephritis and granular immunoglobulin or complement deposition.

Historical note: The Raji cell assay was used to detect circulating immune complexes. It is a laboratory assay, not a disease example or a hypersensitivity reaction in itself.

Oral and Dental Relevance

Systemic Lupus Erythematosus

Oral lesions may include recurrent ulcers, erythematous plaques or lichenoid-appearing lesions.
Patients may have anaemia, leukopenia or thrombocytopenia.
Lupus nephritis may influence medication selection and dosage.
Corticosteroids, antimalarials, biologic agents or other immunosuppressive drugs may alter infection and healing risk.
Review disease activity, blood counts and renal status before significant invasive treatment when clinically indicated.

IgA Vasculitis and Other Vasculitides

Assess for renal involvement, hypertension and current immunosuppressive treatment.
Purpura or mucosal bleeding may indicate platelet, vascular or systemic disease requiring medical review.
Active systemic vasculitis may warrant postponement of elective invasive dental treatment.

Medication-Related Reactions

A delayed fever, rash and arthralgia following medication exposure may suggest serum sickness or a serum sickness-like reaction.
Record the timing, medication, systemic features and previous exposures accurately.
Do not classify every delayed drug rash as Type III hypersensitivity.

Distinction from Anaphylaxis

Type III reactions are not usually immediate reactions occurring within minutes of dental treatment.
Sudden airway compromise, bronchospasm or hypotension should be managed as suspected anaphylaxis rather than serum sickness.
Anaphylaxis is a Type I hypersensitivity reaction and requires immediate emergency management.

Common Classification Errors

Type III differs from Type II by the location of the antigen. Type II antibodies bind fixed antigens on cells or within tissues. Type III antibodies initially bind soluble antigens, forming complexes that later deposit.

Not every glomerulonephritis is Type III. Post-streptococcal and lupus glomerulonephritis are immune-complex mediated. Anti-GBM disease is a Type II reaction against a fixed basement-membrane antigen.

Hypersensitivity pneumonitis is not purely Type III. Acute disease includes immune-complex mechanisms, while cellular Type IV mechanisms are important, especially in chronic disease.

Polyarteritis nodosa is not universally an immune-complex disease. Immune-complex deposition is particularly relevant to hepatitis B-associated cases.

Reactive arthritis is not a standard Type III example. It is a post-infectious inflammatory arthritis in which innate and T-cell-mediated mechanisms are more important than deposited circulating immune complexes.

The Schick test is not a classic example of Type III hypersensitivity. It historically assessed susceptibility to diphtheria toxin and should not be memorized as a core Type III disease.

Type 2 lepra reaction does not mean Type II hypersensitivity. Erythema nodosum leprosum is termed a Type 2 leprosy reaction but is classically associated with a Type III immune-complex mechanism.

Recall Mnemonic

SHARP

S Serum sickness and systemic lupus erythematosus
H Hypersensitivity pneumonitis and IgA vasculitis, historically called Henoch–Schönlein purpura
A Arthus reaction
R Rheumatoid arthritis, in which immune complexes contribute but are not the sole mechanism
P Post-streptococcal glomerulonephritis and hepatitis B-associated polyarteritis nodosa

Use the mnemonic selectively. The strongest classical examples are Arthus reaction, serum sickness, SLE and immune-complex glomerulonephritis. Several other diseases have mixed or context-dependent immune mechanisms.

Examination Pearls

Type III hypersensitivity is caused by soluble antigen–antibody complexes.
The principal antibody is usually IgG, although IgM may also participate.
Intermediate-sized complexes formed in relative antigen excess are particularly likely to persist and deposit.
Large immune complexes are generally cleared efficiently by macrophages in the liver and spleen.
The principal inflammatory cells are neutrophils.
C5a is a potent neutrophil chemotactic and activating factor.
Complement consumption may produce reduced serum C3 and C4.
Arthus reaction is localized; serum sickness is systemic.
Arthus reactions usually develop within approximately 4–12 hours in a previously sensitized individual.
Classical serum sickness commonly develops approximately one to two weeks after a first antigen exposure.
Immunofluorescence generally shows a granular or lumpy-bumpy pattern.
Type II involves antibody binding to a fixed target; Type III involves immune complexes formed against soluble antigens.
SLE is a prototype systemic immune-complex disease.
Hypersensitivity pneumonitis involves both Type III and Type IV mechanisms.

Rapid Recall

Type III = soluble antigen + antibody complex. Persistent immune complexes deposit in tissues, activate complement and recruit neutrophils. The characteristic outcomes are vasculitis, glomerulonephritis and arthritis. Remember: Arthus is localized, serum sickness is systemic, and immunofluorescence is granular.

Immunology · High-Yield Notes

Type IV Hypersensitivity

A delayed, T-cell–mediated reaction in which sensitized CD4⁺ T cells produce inflammatory cytokines or CD8⁺ cytotoxic T cells directly destroy antigen-bearing target cells.

Type IV hypersensitivity is also called delayed-type or cell-mediated hypersensitivity . Unlike Types I, II and III, the principal effector mechanism does not depend on antibody. Tissue injury is produced by activated T lymphocytes, inflammatory cytokines, recruited leukocytes and direct cytotoxicity.

Principal mediator Sensitized T lymphocytes

Antibody involvement Not required

Major pathways CD4 inflammation and CD8 cytotoxicity

Classic timing Peaks at 48–72 hours

Core Immunological Sequence

Initial antigen exposure Antigen presentation T-cell sensitization Memory T-cell formation Re-exposure Cytokine-mediated inflammation or cytotoxicity

Sensitization and Elicitation

Type IV hypersensitivity develops in two stages. The first exposure generates antigen-specific memory T cells; a later exposure activates these cells and produces tissue injury.

Sensitization Phase

Initial exposure

Antigen is captured and processed by an antigen-presenting cell, commonly a dendritic cell.
Antigenic peptides are presented to naïve T cells through major histocompatibility complex molecules.
CD4⁺ T cells differentiate into functional subsets such as TH1, TH17 or TH2 cells, depending on the cytokine environment.
Antigen-specific CD8⁺ T cells may differentiate into cytotoxic T lymphocytes.
Clonal expansion produces long-lived memory T cells.

Elicitation Phase

Re-exposure

Memory T cells recognize the previously encountered antigen.
Activated T cells release cytokines and chemokines or directly attack antigen-bearing cells.
Macrophages, monocytes, eosinophils or neutrophils are recruited according to the dominant T-cell pathway.
Cellular recruitment and activation require time, explaining the characteristic delayed onset.
Persistent antigen exposure may lead to chronic inflammation, fibrosis or granuloma formation.

Important distinction: The patient may be clinically unaffected during sensitization. The inflammatory reaction develops when memory T cells respond to subsequent antigen exposure.

CD4⁺ T-Cell–Mediated Inflammation

TH1–Macrophage Pathway

Classical delayed hypersensitivity

Antigen-presenting cells produce cytokines, including IL-12, which favour TH1 differentiation. On re-exposure, sensitized TH1 cells recognize antigen and release inflammatory mediators.

IFN-γ: activates macrophages and increases their microbicidal activity.

TNF and chemokines: promote endothelial activation and recruitment of monocytes and other leukocytes.

IL-2: supports T-cell proliferation and cellular immune responses.

Activated macrophages: release cytokines, proteases, reactive oxygen species and other mediators that may injure surrounding tissue.

TH17 Pathway

Neutrophil-rich inflammation

TH17 cells contribute to several chronic inflammatory and autoimmune reactions.

IL-17: stimulates epithelial, endothelial and stromal cells to produce chemokines that recruit neutrophils.

IL-22: influences epithelial barrier function and local inflammatory responses.

Clinical consequence: persistent neutrophilic inflammation and tissue damage may occur when the response is excessive or poorly controlled.

Correction of a common error: IL-12 is mainly produced by antigen-presenting cells and helps drive TH1 differentiation. It should not be listed simply as a cytokine released by sensitized TH1 cells during re-exposure.

CD8⁺ T-Cell–Mediated Cytotoxicity

Cytotoxic T lymphocytes recognize antigenic peptides presented by MHC class I molecules on target cells. They then induce apoptosis through two principal pathways.

Perforin–Granzyme Pathway

Cytotoxic T cells release perforin and granzymes at the immunological synapse.

Perforin facilitates delivery of granzymes into the target cell.
Granzymes activate intracellular apoptotic pathways.
The target cell dies without requiring antibody or complement.

Fas–Fas Ligand Pathway

Fas ligand on the activated cytotoxic T cell binds Fas on the target cell.

Fas signalling activates caspases.
The target cell undergoes programmed cell death.
This pathway contributes to immune regulation as well as pathological cytotoxicity.

Important targets: virus-infected cells, tumour cells, transplanted cells and self-cells displaying autoantigens may be attacked by cytotoxic T lymphocytes.

Granulomatous Inflammation

Why Granulomas Form

Granulomas develop when an antigen or organism persists and cannot be eliminated effectively. Continued TH1 activation causes sustained macrophage recruitment and activation.

Macrophages enlarge and become epithelioid histiocytes.
Some macrophages fuse to form multinucleated giant cells.
A surrounding rim of lymphocytes and fibrosis may develop.

Important Examples

Tuberculosis
Tuberculoid leprosy
Some deep fungal infections
Sarcoidosis
Foreign-body reactions, although these are not always antigen-specific hypersensitivity reactions

Protective and harmful: Granulomas may contain a persistent organism, but prolonged macrophage activation can also cause tissue destruction, fibrosis and loss of organ function.

Modern Type IV Subclassification

Type IV reactions can be subdivided according to the dominant T-cell population, cytokines and recruited effector cells. Overlap between subtypes is common.

Type IVa

TH1 and Macrophages

Dominated by TH1 cytokines, particularly IFN-γ, with monocyte and macrophage activation.

Tuberculin reaction
Contact dermatitis
Granulomatous inflammation

Type IVb

TH2 and Eosinophils

TH2 cells release cytokines such as IL-4, IL-5 and IL-13, promoting eosinophilic inflammation.

DRESS syndrome
Some maculopapular drug eruptions
Eosinophil-rich delayed reactions

Type IVc

Cytotoxic T Cells

CD4⁺ or CD8⁺ cytotoxic cells induce apoptosis through perforin, granzymes and Fas ligand.

Stevens–Johnson syndrome
Toxic epidermal necrolysis
Some contact and drug reactions

Type IVd

T Cells and Neutrophils

T-cell-derived chemokines and growth factors promote neutrophil recruitment and activation.

Acute generalized exanthematous pustulosis
Neutrophilic delayed drug reactions

Exam note: The IVa–IVd system is especially useful for classifying delayed drug hypersensitivity. Many clinical reactions contain more than one pathway.

Classic Clinical Examples

Tuberculin Skin Reaction

Classic delayed reaction

Intradermal tuberculin antigen is recognized by previously sensitized memory T cells.

TH1 cells release cytokines.
Monocytes and macrophages accumulate.
Induration is assessed rather than erythema.
The test is generally read at 48–72 hours.

Allergic Contact Dermatitis

Hapten-mediated reaction

Small chemicals penetrate the skin, bind host proteins and form antigenic complexes that activate sensitized T cells.

Nickel
Chromates
Fragrances and preservatives
Rubber accelerators
Acrylates and methacrylates

Cell-Mediated Autoimmune Disease

Type 1 diabetes mellitus Autoreactive T cells destroy pancreatic beta cells.

Multiple sclerosis T-cell–driven inflammation contributes to CNS demyelination.

Hashimoto thyroiditis T-cell-mediated follicular destruction occurs alongside autoantibody responses.

Transplantation

Acute cellular rejection Recipient T cells recognize donor alloantigens and injure graft parenchymal and vascular cells.

Chronic rejection T-cell, antibody and vascular mechanisms contribute to progressive fibrosis and graft dysfunction.

Type 1 Lepra Reaction

A reversal reaction in borderline leprosy caused by a change in cell-mediated immunity against mycobacterial antigens.

Existing lesions become inflamed.
Nerve inflammation may cause acute neurological damage.
It differs from erythema nodosum leprosum, which has an important immune-complex mechanism.

Tumour and Antiviral Immunity

CD8⁺ cytotoxic T cells recognize and destroy virus-infected or malignant cells.

These are physiologically protective cell-mediated immune responses. They illustrate Type IV effector mechanisms but are not necessarily pathological hypersensitivity reactions.

Diagnostic Tests

Delayed skin testing

Tuberculin Skin Test

Purified protein derivative is injected intradermally. The diameter of palpable induration is measured at 48–72 hours.

A positive result indicates prior immune sensitization and must be interpreted using the patient’s risk factors and applicable clinical criteria.

Contact allergy

Patch Testing

Suspected allergens are applied to intact skin under occlusion and evaluated for a delayed eczematous reaction.

Patch testing is used for allergic contact dermatitis and is different from skin-prick testing used for immediate IgE-mediated allergy.

Tissue examination

Biopsy

Histology depends on the condition but may demonstrate lymphocytic inflammation, spongiosis, interface dermatitis, granulomas or cytotoxic epithelial injury.

Drug hypersensitivity

Clinical Assessment

Diagnosis depends heavily on the medication timeline, morphology of the eruption, organ involvement and exclusion of alternative causes.

Specialist-directed patch testing, delayed intradermal testing or other investigations may be considered in selected cases.

Test distinction: Skin-prick testing assesses immediate IgE-mediated reactivity. Patch testing assesses delayed T-cell–mediated contact hypersensitivity.

Oral and Dental Relevance

Dental Material Reactions

Acrylates and methacrylates in resins, adhesives and restorative materials may cause occupational contact dermatitis.
Nickel, cobalt, chromium and other metals may produce delayed contact reactions in sensitized individuals.
Rubber-processing chemicals in gloves can cause Type IV contact dermatitis.
Irritant contact dermatitis is more common than true allergic contact dermatitis and is not an immune hypersensitivity reaction.

Latex: Immediate versus Delayed

Immediate allergy to natural rubber latex proteins is typically an IgE-mediated Type I reaction.
Delayed eczematous reactions are often caused by rubber additives and accelerators and represent Type IV hypersensitivity.
The clinical history must distinguish immediate urticaria, respiratory symptoms or anaphylaxis from delayed dermatitis.

Allergic Contact Stomatitis

Potential triggers include cinnamon flavouring, fragrance compounds, oral hygiene products and dental materials.
Features may include burning, erythema, oedema, ulceration, desquamation or lichenoid changes.
Diagnosis requires correlation between the distribution of lesions and exposure to the suspected agent.
Improvement after removal of the suspected allergen supports the diagnosis.

Oral Lichenoid Reactions

Localized lichenoid lesions may occasionally occur adjacent to dental restorations or in association with medicines.
T-cell–mediated epithelial injury is an important pathological mechanism.
Oral lichen planus is a complex immune-mediated disorder and should not be reduced to a simple material allergy.
Biopsy may be required when the diagnosis is uncertain or lesions are persistent.

Severe Delayed Drug Reaction: Red Flags

Urgent medical assessment is required when a delayed drug eruption is accompanied by:

Painful oral, ocular or genital mucosal erosions
Skin blistering, epidermal detachment or widespread tenderness
Facial oedema, fever or extensive rash
Lymphadenopathy or systemic illness
Hepatic, renal, haematological or respiratory involvement

Severe reactions such as Stevens–Johnson syndrome, toxic epidermal necrolysis and DRESS are not managed as routine localized oral allergy.

Common Classification Errors

Type IV is not antibody-mediated. The principal effectors are sensitized T lymphocytes and recruited inflammatory cells.

“Delayed” does not always mean exactly 48–72 hours. The tuberculin reaction typically peaks within this period, but delayed drug reactions may develop over several days or weeks.

IL-12 is mainly an antigen-presenting-cell cytokine. It promotes TH1 differentiation rather than serving as a principal TH1 effector cytokine released during re-exposure.

Cytotoxic T cells are not responsible for ADCC. CD8⁺ T cells kill through antigen recognition on MHC class I. Antibody-dependent cellular cytotoxicity is typically mediated by NK cells through Fc receptors.

Not all graft rejection is purely Type IV. Acute cellular rejection is strongly T-cell mediated, whereas antibody and vascular mechanisms also contribute to other rejection patterns.

Sarcoidosis and Crohn disease are immunologically complex. Granulomatous and T-cell-mediated mechanisms are important, but these conditions should not be presented as isolated, uncomplicated hypersensitivity reactions.

Type 1 and Type 2 lepra reactions are different. Type 1 lepra reaction is predominantly cell mediated. Type 2 lepra reaction, or erythema nodosum leprosum, has an important immune-complex component.

Rapid Recall Framework

“T Cells Take Time”

T Cells CD4⁺ helper T cells and CD8⁺ cytotoxic T cells are the principal mediators.

Take Cellular recruitment and cytokine production require time; classical reactions peak around 48–72 hours.

Time Re-exposure activates memory T cells, leading to inflammation, cytotoxicity or granuloma formation.

Examination Pearls

Type IV hypersensitivity is mediated by T lymphocytes rather than antibodies.
Classical delayed-type hypersensitivity usually peaks at 48–72 hours.
Antigen presentation to CD4⁺ T cells occurs through MHC class II.
CD8⁺ cytotoxic T cells recognize antigen presented through MHC class I.
TH1-derived IFN-γ is a major activator of macrophages.
TH17 cells recruit neutrophils principally through IL-17-driven chemokine production.
Cytotoxic T cells induce apoptosis through perforin–granzyme and Fas–Fas ligand pathways.
Persistent TH1–macrophage activation may produce granulomatous inflammation.
The tuberculin reaction and allergic contact dermatitis are classical examples.
The tuberculin skin test is evaluated by measuring induration rather than erythema.
Patch testing assesses delayed contact hypersensitivity; skin-prick testing assesses immediate IgE-mediated allergy.
Type IVa involves TH1 cells and macrophages; IVb involves TH2 cells and eosinophils.
Type IVc involves cytotoxic T cells; IVd involves neutrophilic inflammation.
Type 1 diabetes mellitus is principally a T-cell-mediated autoimmune disease.
Immediate latex-protein allergy is Type I, whereas delayed allergy to rubber additives is Type IV.

Rapid Recall

Type IV = T cells, no antibody. Initial exposure generates sensitized memory T cells. Re-exposure activates CD4⁺ cytokine-mediated inflammation or CD8⁺ direct cytotoxicity. The classic reaction peaks at 48–72 hours, while persistent TH1–macrophage activation may produce granulomas.

Immunology · Classification Clarification

“Type V” Hypersensitivity

A historical, non-standard term sometimes used for antibodies that stimulate or block cell-surface receptors.

Not a separate Gell and Coombs category: The accepted classification contains Types I–IV. Reactions historically labelled “Type V” are classified as antibody-mediated functional disturbances within Type II hypersensitivity.

True classification Type II hypersensitivity

Principal mediator IgG autoantibodies

Main effect Altered receptor function

Autoantibody Cell-surface receptor Stimulation or blockade Abnormal cellular function

Receptor stimulation

Graves Disease

Autoantibodies stimulate the TSH receptor, increasing thyroid-hormone production.

Receptor dysfunction

Myasthenia Gravis

Autoantibodies impair acetylcholine-receptor function at the neuromuscular junction, producing fatigable muscle weakness.

Common error: Antibody-dependent cellular cytotoxicity is not “Type V.” ADCC is an effector mechanism of Type II hypersensitivity.

Exam Takeaway

Do not classify Type V as an independent reaction. Graves disease, myasthenia gravis and other receptor-altering antibody disorders belong under Type II.