Allergy and Hypersensitivity Types

A hypersensitivity reaction is an exaggerated or misdirected immune response. An allergy is immune-mediated; an intolerance is not. Nausea after an antibiotic, for example, may be unpleasant but does not by itself indicate an allergy. Confusing the two can unnecessarily restrict future treatment, while missing a true allergy can expose the patient to a life-threatening reaction. The classic Gell and Coombs framework describes four mechanisms: - Type I, immediate: An allergen cross-links IgE on mast cells and basophils. Mediators such as histamine cause vasodilation, increased vascular permeability, bronchoconstriction, pruritus, urticaria, angioedema, gastrointestinal symptoms, and potentially shock. Food, insect venom, and some medication reactions may follow this pattern. - Type II, antibody-mediated cell injury: IgG or IgM binds to antigens on cell surfaces or circulating cells. Complement activation or cellular destruction can produce haemolysis, thrombocytopenia, or other cytopenias. An acute haemolytic transfusion reaction is a high-risk example. - Type III, immune-complex disease: Antigen-antibody complexes deposit in tissues and activate complement. The resulting inflammation may produce serum sickness, vasculitis, joint symptoms, or renal injury. - Type IV, delayed cell-mediated: Sensitized T cells...