antigen-antibody reaction, a process in which antibodies bind to antigens to form antigen-antibody complexes. These complexes may render toxic antigens harmless (neutralization), agglutinize antigens on the surface of microorganisms, or activate the complement system by exposing the complement binding sites on antibodies. Certain complement protein molecules immediately bind to these sites and trigger the activity of the other complement protein molecules, which cause antigen-bearing cells to lyse. Antigen-antibody reactions may start immediately with antigen contact or as much as 48 hours later. They normally produce immunity but may also be responsible for allergy, autoimmunity, and fetomaternal hematologic incompatibility. In the immediate allergic response, the antigen-antibody reaction activates certain enzymes and causes an imbalance between those enzymes and their inhibitors. Simultaneously released into the circulation are several pharmacologically active substances, including acetylcholine, bradykinin, histamine, immunoglobulin G, and leukotaxine. See also allergen, allergic reaction, humoral immunity, serum sickness.
