Naturally occurring antibodies that react with protozoan parasites.

In this paper, Eiji Konishi reviews general features of naturally occurring (natural) antibodies that react with protozoan parasites. Several functions of natural antibodies have been identified in relation to their multireactivity, but reports on protozoan infection have dealt mainly with the role of natural antibodies in the innate immunity of the host, These antibodies lyse cells in the presence of complement and have opsonizing activity, in vitro. Studies of their origin have shown the possibilities of (1) continuous polyclonal stimulation by gastrointestinal bacteria, and (2) there being multireactive antibodies secreted by CD5(+) B cells. The protective functions of natural antibodies are important in the interpretation of the host range, the mode of infection, and the course of the disease of certain protozoan parasites.