Hypersensitivity Types and Their Mechanisms

Type I - lgE - Soluble antigen

 - Allergen-specific lgE antibodies bind to mast cells via their Fc receptor. When the specific allergen binds to the lgE, cross-linking of lgE induces degranulation of mast cells.

 - Local and systemic anaphylaxis, seasonal hay fever, food allergies, and drug allergies

Type II - IgG or IgM - Cell-bound antigen 

 - IgG or IgM antibody binds to cellular antigen, leading to complement activation and cell lysis. IgG can also mediate ADCC with cytotoxic T cells, natural killer cells, macrophages, and neutrophils.

 - Red blood cell destruction after transfusion with mismatched blood types or during hemolytic disease of the newborn.

Type Ill - IgG and IgM - Soluble antigen

 - Antigen-antibody complexes are deposited in tissues. Complement activation provides inflammatory mediators and recruits neutrophils. Enzymes released from neutrophils damage tissue.

 - Post-streptococcal glomerulonephritis, rheumatoid arthritis, and systemic lupus erythematosus

Type IV - T cells - Soluble or cell-bound antigen

 - TH1 cells secrete cytokines, which activate macrophages and cytotoxic T cells.

 - Contact dermatitis, type I diabetes mellitus, and multiple sclerosis



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