Association of homozygous C4B deficiency with bacterial meningitis.

The fourth component of complement (C4) is encoded by two separate but linked loci (C4A and C4B), each of which produces functionally active C4. Although C4A and C4B share certain antigenic and functional characteristics that identify them as C4, they differ with respect to other structural and functional properties. For example, C4B possesses four times the functional hemolytic activity of C4A. This suggests that homozygous deficiency of C4B might be associated with an increased susceptibility to infection. Forty-six children with bacterial meningitis were examined. Of these, 5 (10.9%) were homozygous deficient for C4B versus 7 (3.1%) of 223 controls (P = 0.038). There was no relation between the prevalence of heterozygous C4B deficiency and meningitis or between the prevalence of either homozygous or heterozygous C4A deficiency and meningitis. These results suggest that homozygous C4B deficiency is a relatively common immunodeficiency disorder that is clinically significant and predisposes children to bacterial meningitis.