Complement deficiency: predisposing factor to autoimmune syndromes. 
The recognition of the association between complete and partial complement (C) deficiencies and immune complex (IC) mediated diseases, especially SLE, is of clinical and etiopathologic interest.  From studies of sera deficient in C1, C4, C2 or C3, the crucial role of these components of the classical pathway in promoting the solubility and clearance of IC has been elucidated.  Although complete deficiency of C1, C4 C2 or C3 is rare, partial C4 deficiency also appears to be a common predisposing factor for the development of SLE, with complete C4A deficiency (C4A null) being present in 10 to 15% and heterozygous C4A deficiency present in 50 to 80% of caucasian SLE patients.  Most importantly, there is an obvious pathophysiologic relationship between the function of C relative to IC processing and the syndromes which result from C deficiency.  Thus, SLE is a prototype for disease characterized by excessive quantities of inappropriately deposited IC.  More subtle C component and receptor deficiencies are likely to be predisposing factors for SLE and related autoimmune diseases.