Inherited deficiencies of complement in rheumatic diseases.

Inherited complement deficiencies are rare but occur with increased frequency in systemic lupus erythematosus. This selective review summarizes the fundamentals of complement activation and biology and focuses on recent findings. system lupus erythematosus is highly associated with homozygous C1q deficiencies and with C4A null allotypes, and homozygous C2 and C3 deficiencies are less uniformly associated with autoimmune disease. This hierarchy of disease associations suggests the importance of an intact early classic activation pathway in preventing systemic lupus erythematosus. Assays of complement activation products may have greater sensitivity and specificity for systemic lupus erythematosus disease activity and lupus flares.