A term used in serology and immunology to refer to a substance that enhances the phagocytosis of bacteria by leukocytes. As originally proposed by A. E. Wright and E. R. Douglas, the term denoted a thermolabile, relatively nonspecific substance present in normal sera. In modern usages, opsonin is more generally synonymous with the bacteriotropin of F. Neufeld and coworkers, a relatively thermostable antibody, increased in amount during specific immunization, that renders the corresponding bacterium more susceptible to phagocytosis. There is evidence that this action can be promoted to some extent by antibody alone, but that it is substantially increased by the further addition of the thermolabile complement system. Opsonic activity can be displayed by antibodies that also give precipitation, agglutination, lytic, and neutralization reactions.