the binding of an antibody to its particular antigen as part of the body’s natural defense against the introduction of a foreign substance. The bound antibody renders the foreign substance more susceptible to degradation by immune cells, for example, engulfment by phagocytic cells. Once the individual’s immune system has developed antibodies to fight a certain type of antigen, the antibodies can be mobilized quickly to destroy any repeated invasion by the antigen; thus, the first antigen attack induces immunity to future attacks. See also immune response.