17.6: Hypersensitivity Reactions: Cytolytic Reactions

Type II hypersensitivity involves IgG and IgM antibodies targeting cell surface antigens, leading to cell destruction. This can occur through complement activation, antibody-dependent cell-mediated cytotoxicity (ADCC), or acting as opsonins for phagocytosis. When excessive, these reactions cause significant tissue damage.

Drug-induced hemolytic anemia is a common example, where drugs like penicillin or cephalosporins bind to red blood cells, forming drug-protein complexes. These complexes trigger the formation of antibodies, leading to complement-mediated lysis of red blood cells and anemia. Withdrawal of the drug usually resolves the condition. Hemolytic anemia can also result from other drugs like methyldopa, used in hypertension, causing antibodies to target red blood cells, leading to their destruction. The Coombs test can detect this reaction, indicating the presence of antibodies against red blood cell surfaces.

Type II hypersensitivity, or cytolytic reactions, involves IgG and IgM antibodies binding to antigens present on the surface of host cells. This binding can trigger the complement system or recruit cytotoxic cells, ultimately resulting in targeted cell death.

In some cases, the antibodies can also tag the cell for clearance through opsonization—a process where opsonins like IgG mark targets to be recognized by phagocytes. The phagocytes then engulf and digest the marked cells.

A classic example of this type of hypersensitivity is hemolytic anemia induced by penicillin. In this condition, the drug binds to red blood cells, forming a drug–protein complex. Antibodies bind to these drug–protein complexes, activating complement components and causing lysis of the RBCs.