The immunological responses in hosts infected with Trypanosoma lewisi were examined during the course of infection and after recovery. Peak antibody levels coincided with the time of parasite elimination, but remained significantly elevated for over one year after the end of the infection The antigen repertoire recognized by antibodies demonstrated that some were revealed only by sera taken during the infection, and other antigens were revealed for the first time only by post-recovery sera. Immunomodulatory protective and suppressive factors were demonstrated in the plasma of irradiated, infected rats. These factors were identified as parasite-derived exoantigens which are shed in vivo and in vitro; exoantigens are complexes of proteins, lipids and polysaccharides and are membrane-surface coat associated, as shown by phase-partitioning and surface-labeling studies. The suppressive activity of the exoantigens was dose-dependent, probably mediated by a suppressor substance(s) produced by macrophages that subsequently inhibits production of interleukin 2 by T helper cells.

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