Hypersensitivity Reactions — ACID

Type I — ANAPHYLACTIC (IgE-mediated) • Mast cell degranulation → histamine release • Examples: anaphylaxis, allergic rhinitis, asthma, atopic dermatitis, food allergies • Tests: skin prick, RAST (specific IgE) Type II — CYTOTOXIC (IgG/IgM, complement) • Antibody binds to fixed antigen → complement → cell lysis • Examples: AIHA, Goodpasture, Graves, Myasthenia gravis, Pemphigus, ITP, transfusion reactions • Tests: direct/indirect Coombs Type III — IMMUNE COMPLEX (IgG + antigen complexes deposit) • Examples: SLE, post-strep GN, serum sickness, Henoch-Schönlein, Polyarteritis nodosa, Arthus reaction • Mnemonic: SHARP Type IV — DELAYED (T-cell mediated, no antibody) • CD4 (DTH) or CD8 (cytotoxic) • Examples: contact dermatitis (poison ivy, nickel), TB skin test (PPD), graft rejection, T1DM, MS, RA, IBD Note: many autoimmune diseases involve multiple types simultaneously.