Deficiencies of individual complement proteins may be accompanied by SLE or related syndromes. Deficiencies of the classic activation pathway are often involved. In cases of C4 and C2 deficiency, there is evidence that this association occurs more frequently than would be expected by chance. The clinical picture differs from classic SLE. There is an increased frequency of skin involvement, a decreased frequency of renal disease, low or absent levels of antibody to native DNA, and increased levels of anti-Ro (SS-A). The mechanism for the association probably involves the effects of C3 and C4 on the precipitation of immune complex solubility, or on their processing through cell surface c4b/c3b receptors on phagocytes. Disseminated or recurrent Neisseria infections are common in patients lacking the constituents of the terminal MAC that are important in killing these organisms.