Evidence has accumulated during the last decade that immunopathological mechanisms are involved in the pathogenesis of lesions in several parasitic infections. Although both humoral and cellular mechanisms may be involved, considerable progress has been made in the understanding of lesions initiated (and perpetuated?) by humoral components, mainly immune complexes (IC) and complement, in protozoal infections, particularly in malaria. I would like to stress that formation of IC, i.e., binding of antibody with relevant antigen, is a physiological mechanism and protective function of antibody, and, therefore, that the antigen-antibody complexes formed are part of the defense system. Most of the IC formed are removed by physiological routes and their presence in circulation and/or in phagocytic cells should not always be regarded as a pathological situation. However, due to the multivalent properties of IC, they may initiate harmful effects leading either to tissue injury or to impairment of otherwise effective immunological mechanisms.