The pathobiology of the terminal complement complexes

C5b and the other late-acting complement components can assemble the two terminal complexes (TCC) C5b-9 and SC5b-9. In addition to the lytic effects of C5b-9 it has been demonstrated that sublytic amounts of C5b-8 or C5b-9 can stimulate several important cellular activities. These effects may be important to explain the role of C5b-9 in the production and progression of several pathological conditions that has been demonstrated in experimental models of disease. With the help of antibodies that specifically recognize C9 neoantigens, deposits of TCC have been identified in human tissues, not only in immunological diseases but also in certain nonimmunological diseases. In the latter it has been shown that often there is no concordance between deposits of TCC and those of immunoglobulins and C3. Methods for measuring SC5b-9 in biological fluids have also been developed. Normal plasma was found to have low levels of SC5b-9. Increased plasma levels of SC5b-9 have been observed during the active phase of SLE nephritis, in certain infections and during cardiopulmonary bypass. Increased levels were also found in the cerebrospinal fluid of patients with inflammatory diseases of the central nervous system and in the synovial fluid of patients with rheumatoid arthritis. Autoantibodies to C9 neoantigens in plasma of certain patients with autoimmune, infectious or neoplastic diseases have recently been recognized. Additional work is needed to better delineate the potential usefulness of these findings for diagnosis and evaluation of disease activity.