Complement is part of the innate immune system. Its major function is recognition and elimination of pathogens via direct killing and/or stimulation of phagocytosis. Activation of the
complement system is, however, also involved in the pathogenesis of the systemic
autoimmune diseases. Activation via the classical pathway has long been recognized in
immune complex-mediated diseases such as cryoglobulinemic
vasculitis and
systemic lupus erythematosus (SLE). In SLE, the role of
complement is somewhat paradoxical. It is involved in
autoantibody-initiated tissue damage on the one hand, but, on the other hand, it appears to have protective features as hereditary deficiencies of classical pathway components are associated with an increased risk for SLE. There is increasing evidence that the alternative pathway of
complement, even more than the classical pathway, is involved in many systemic
autoimmune diseases. This is true for
IgA-dominant Henoch Schönlein
Purpura, in which additional activation of the
lectin pathway contributes to more severe disease. In
anti-neutrophil cytoplasmic antibody (
ANCA)-associated vasculitis the
complement system was considered not to be involved since
immunoglobulin deposition is generally absent in the lesions. However, recent studies, both in human and animal models, demonstrated complement activation via the alternative pathway as a major pathogenic mechanism. Insight into the role of the various pathways of
complement in the systemic
autoimmune diseases including the
vasculitides opens up new ways of treatment by blocking effector pathways of
complement. This has been demonstrated for
monoclonal antibodies to C5 or C5a in experimental
anti-phospholipid antibody syndrome and
ANCA-associated vasculitis.