The complement system represents an important nonspecific skin defense mechanism. Its activation leads to the generation of products that not only help to maintain normal host defenses but also mediate inflammation and tissue injury. Proinflammatory products of complement include large fragments of C3 with opsonic and cell-stimulatory activities (C3b and C3bi), low molecular weight anaphylatoxins (C3a, C4a, and C5a), and membrane attack complex. Among them C5a or its degradation product C5a des Arg seems to be the most important mediator because it exerts a potent chemotactic effect on inflammatory cells. Intradermal administration of C5a anaphylatoxin induces skin changes quite similar to those observed in cutaneous hypersensitivity vasculitis that occurs through immune complex-mediated complement activation. Complement activation is involved in the pathogenesis of the inflammatory changes in autoimmune bullous dermatoses. In pemphigus complement activation by pemphigus antibody in the epidermis seems to be responsible for the development of characteristic inflammatory changes termed eosinophilic spongiosis. In bullous pemphigoid (BP) interaction of basement membrane zone antigen and BP antibody leads to complement activation that seems to be related to leukocytes lining the dermoepidermal junction. Resultant anaphylatoxins not only activate the infiltrating leukocytes but also induce mast cell degranulation which facilitates dermoepidermal separation and eosinophil infiltration. Similar complement activation seems to play a more direct role in the dermoepidermal separation noted in epidermolysis bullosa acquisita and herpes gestationis. Anaphylatoxin generation via the alternative pathway activation under light irradiation is implicated in the development of the immediate erythematous phototoxic reactions induced by such well-known chemicals as porphyrin, chlorothiazide, demethylchlortetracycline, and chlorpromazine.(ABSTRACT TRUNCATED AT 250 WORDS)