The association between C1q and autoimmune diseases such as rheumatoid arthritis and systemic lupus erythematosus (SLE) is well established. Deficiency in C1q is considered to be a strong susceptibility factor and is corroborated by the fact that > or = 92% of the known cases of hereditary deficiency in C1q develop rheumatic disease. Furthermore, the observation of the presence of high-affinity autoantibodies against C1q antibodies in patients with SLE provides a strong correlation between these antibodies and the inflammatory processes that occur in this disease. Recent evidence using C1q-deficient mice has shown the presence of glomerulonephritis with immune deposits and a large number of apoptotic bodies in the diseased glomeruli suggesting a defect in the clearance of apoptotic cell by macrophages and dendritic cells (DCs). Although these data are consistent with the hypothesis that C1q deficiency may induce a generalized failure to clear immune complexes and apoptotic cells, this concept alone cannot wholly explain why individuals with C1q deficiency are prone to develop SLE. Therefore, C1q alone or in conjunction with other surface molecules must play a much more fundamental role in immunoregulation, especially those processes that regulate T cell function and tolerance. In support of this hypothesis is the finding that C1q causes inhibition of mitrogen-induced T cell-proliferative response by interaction with C1q receptors. Furthermore, macrophages and possibly DCs not only synthesize but also display C1q as a type II cell surface molecule, especially at sites of inflammation. Although it is not yet known what role the surface-expressed C1q plays, it is tempting to assume that it plays a role in the priming of naïve T cells by DCs. This work will review the current concepts of the role of C1q and C1q receptors in autoimmunity.