immunity

immunity The state of relative insusceptibility of an animal to infection by disease-producing organisms or to the harmful effects of their poisons (toxins). Immunity depends on the presence in the blood of antibodies and white blood cells (lymphocytes), which produce an immune response. Inherited (natural or innate) immunity is that with which an individual is born. Acquired immunity is of two types, active and passive. Active immunity arises when the body produces antibodies against an invading foreign substance (antigen), either through infection or immunization; this type of immunity may be humoral, in which B lymphocytes produce free antibodies that circulate in the bloodstream (see B cell), or cell-mediated, caused by the action of T lymphocytes (see T cell). Passive immunity is induced by injection of serum taken from an individual already immune to a particular antigen; it can also be acquired by the transfer of maternal antibodies to offspring via the placenta or breast milk (see colostrum). Active immunity tends to be long-lasting; passive immunity is short-lived. See also autoimmunity.