SUMMARY

IgG (Immunoglobulin G)

• Has 2 heavy and 2 light chains.
• Sedimentation coefficient: 7S.
• Main antibody in secondary immune response.
• Crosses placenta → provides neonatal immunity.
• Most abundant in serum (80%).
• Half-life: maximum among all Igs.
• Present in milk (along with IgA).
• Fixes complement via classical and alternate pathways.
• Least carbohydrates among Igs.

IgM (Immunoglobulin M)

• Largest, pentamer with J-chain, 19S macroglobulin.
• First antibody produced in primary immune response.
• Fixes complement via classical pathway.
• Maximum molecular weight, maximum sedimentation coefficient.
• Highly effective in agglutination due to 10 antigen-binding sites.
• Present in serum and on B-cell surface.

IgA (Immunoglobulin A)

• Dimer with J-chain in secretions.
• Secretory component protects against proteolytic enzymes.
• Found in milk, tears, saliva, sweat, respiratory/GIT/UGT secretions.
• Second most abundant Ig (10–13%).
• Only Ig secreted by sero-mucous glands.
• Provides mucosal immunity.
• Present in colostrum → protects neonates.
• Fixes complement via alternate pathway.

IgD (Immunoglobulin D)

• Structure similar to IgG.
• Found on naïve B lymphocytes as membrane receptor.
• Very low serum concentration (30 µg/ml).
• Co-expressed with IgM on immature B cells.
• Fixes complement via alternate pathway.

IgE (Immunoglobulin E)

• Heat labile (only one that isn’t heat stable).
• Involved in allergy, anaphylaxis, and Type I hypersensitivity.
• Binds to mast cells and basophils, causes histamine release.
• Causes vasodilation, bronchial constriction, hypotension.
• Least concentration in serum.
• Fixes complement via alternate pathway.
• Role in asthma, urticaria, hay fever, food allergies.

Functions of Different Immunoglobulins

Immunoglobulin	Function
IgG	Protects body fluids
IgA	Protects body surfaces
IgM	Protects bloodstream
IgE	Mediates allergic reactions
IgD	B cell membrane receptor

Unique Features of Immunoglobulins

• Only Ig not heat stable → IgE
• Only Ig secreted by sero-mucous glands → IgA
• Ig present in milk → IgA and IgG
• Fixes complement via alternate pathway → IgA, IgD, IgG (G4)
• Fixes complement via classical pathway → IgG and IgM
• Maximum serum concentration → IgG
• Least serum concentration → IgE
• Maximum molecular weight → IgM
• Maximum sedimentation coefficient → IgM
• Maximum carbohydrates → IgE
• Maximum half-life → IgG
• Least carbohydrates → IgG
• Minimum half-life → IgE
• Crosses placenta → IgG
• Secretory antibody → IgA
• Mast cell degranulation → IgE
• Naïve B cell receptor → IgD + IgM
• Most abundant overall → IgG
• Most abundant in secretions → IgA
• Natural passive immunity → IgG via placenta, IgA via colostrum