C3 is cleaved by C4b2a, releasing C3a and leaving C3b, the major opsonin of the complement system. C3b also acts on C5 to initiate the MAC (C5b-C9). C3 deficiency results in impaired opsonization and impaired initiation of the final common pathway (MAC). C3 deficiency is extremely rare with only 27 reported cases among 19 families worldwide. Individuals deficient in C3 usually have less than 1% of normal levels of C3 antigenically and functionally. C3 deficiency rarely leads to systemic lupus erythematosus / SLE-like disease, rather it typically results in recurrent pyogenic infections and membrano-proliferative glomerulonephritis / MPGN. Infections are often caused by N. meningitidis, H. influenzae, Enterobacter aerogenes, and Escherichia coli. Respiratory tract infections are prominent, including pneumonia, tonsillitis, sinusitis, and otitis. Renal disease, including MPGN and mesangiocapillary glomerulonephritis, has been seen in 26% of reported cases of C3 deficiency. An SLE-like syndrome with fever, vasculitic skin lesions, and arthritis has been seen in 28% of reported cases of C3 deficiency.
1. Pettigrew H D, et al. (2009). Clinical significance of complement deficiencies. Annals of the New York Academy of Sciences, 1173(1), 108-123.
2. Botto M. (1999). C1q knock-out mice for the study of complement deficiency in autoimmune disease. Experimental and clinical immunogenetics, 15(4), 231-234.