Evolutionary pressures exerted by parasites on the immune system, and vice versa, are surveyed from a speculative viewpoint. New information is presented about the possible channelling of suppression by MHC Class II genes in the mouse, where a novel pattern of dominant unresponsiveness mediated by H-2Ab is described. In addition, the hypothesis is advanced that phosphatidyl-inositol anchorage on the surface of parasites may represent a novel evasion mechanism, in which the spread of the immune response by epitope linkage is inhibited by host phospholipase.