Among the manifold immunologic events which take place during parasitic invasions, production of autoantibodies and immune complexes can play a serious role during infections with African and American trypanosomes. The importance of complement deserves new attention. The increasing level of IgE induced by helminthic infections on a humoral basis seems to be caused by separate worm allergens; its involvement in self-cure phenomena together with processes on cellular levels is discussed. Destructive processes on various development stages of schistosoma and immunological events during cutaneous leishmaniasis are also cell mediated. Variations in the antigenic behaviour of parasites and their immunological mimicry by uptake of substances from the host and their immunosuppressive action are discussed. As reasons for absence of immunity against animal parasites this action can disturb either humoral or cellular immunological procedures or both.