Complement deficiency. Predisposing factor to autoimmune syndromes

The recognition of the association between complete and partial complement (C) deficiencies and immune complex-mediated diseases is of clinical and etiopathologic interest. From studies of sera deficient in C1, C4, C2, or C3, the crucial role of these complement components in promoting the solubility and clearance of immune complexes has been elucidated. Moreover, partial C4 deficiency appears to be a common risk factor for the development of systemic lupus erythematosus, with complete C4A deficiency (C4A null) being present in 10 to 15 percent and heterozygous C4A deficiency present in 50 to 80 percent of patients with systemic lupus erythematosus. Most importantly, there is an obvious pathophysiologic relationship between the function of complement relative to immune complex processing and the disease that results from their deficiency. Systemic lupus erythematosus is characterized by excessive quantities of inappropriately deposited immune complexes. More subtle complement component and receptor deficiencies are likely to be predisposing factors for autoimmune disease. The complement deficiencies provide us with a unique opportunity to investigate the origin and development of immune complex excess syndromes.