Type II HSR
Summary
Type II Hypersensitivity Reaction (HSR) is an antibody-mediated immune response where IgG or IgM antibodies bind directly to antigens on the surface of cells or tissues, leading to cell destruction via complement activation, antibody-dependent cellular cytotoxicity (ADCC), or cellular dysfunction. Classic examples include autoimmune hemolytic anemia, Goodpasture syndrome, and Graves disease.
Detail
Type II hypersensitivity reactions occur when IgG or IgM antibodies target antigens present on the surface of cells (often self-antigens) or extracellular matrix components, triggering tissue damage through several mechanisms: (1) complement-mediated lysis (e.g., membrane attack complex formation causing cell lysis, as in autoimmune hemolytic anemia and transfusion reactions), (2) antibody-dependent cellular cytotoxicity (ADCC) where NK cells or macrophages destroy antibody-coated cells, (3) opsonization leading to phagocytosis (e.g., by splenic macrophages in autoimmune hemolytic anemia), and (4) antibody-mediated cellular dysfunction without direct cell killing, where antibodies bind to receptors and either stimulate or block normal receptor function (e.g., Graves disease with TSH receptor stimulation, myasthenia gravis with blocking of acetylcholine receptors).
Key clinical examples include: Goodpasture syndrome (anti-GBM antibodies attacking type IV collagen in glomerular and alveolar basement membranes), autoimmune hemolytic anemia (antibodies against RBC surface antigens), Graves disease (stimulating antibodies against TSH receptor), myasthenia gravis (blocking antibodies against acetylcholine receptors), Pemphigus vulgaris (antibodies against desmoglein), rheumatic fever (molecular mimicry between streptococcal antigens and cardiac tissue), hemolytic disease of the newborn (Rh incompatibility), and acute transfusion reactions (ABO incompatibility).
Diagnostically, direct and indirect Coombs tests (direct/indirect antiglobulin tests) are used to detect antibody or complement bound to RBCs, useful in diagnosing autoimmune hemolytic anemia and hemolytic disease of the newborn. Understanding the difference between Type II (antibody against fixed cell-surface antigen) versus Type III (antibody-antigen immune complexes deposited in tissues) is a common exam distinction point.
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- Kaplan USMLE Step 1 Immunology Lecture Notes
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