Pathophysiology
Clinical meaning
Hypersensitivity reactions are classified into four types by the Gell and Coombs system based on the immune mechanism involved. Type I (immediate/anaphylactic) is IgE-mediated mast cell degranulation occurring within minutes of allergen exposure, causing urticaria, bronchospasm, and anaphylaxis (e.g., penicillin allergy, food allergies, insect stings). Type II (cytotoxic) involves IgG or IgM antibodies directed against cell surface antigens causing complement-mediated cell destruction (e.g., hemolytic transfusion reactions, hemolytic disease of the newborn, autoimmune hemolytic anemia). Type III (immune complex) involves antigen-antibody complex deposition in tissues causing complement activation and inflammation (e.g., serum sickness, lupus nephritis, rheumatoid arthritis). Type IV (delayed/cell-mediated) is T-cell mediated occurring 24-72 hours after exposure (e.g., contact dermatitis from poison ivy, tuberculin skin test reaction, transplant rejection).