C4B deficiency: a risk factor for bacteremia with encapsulated organisms.

The fourth component of complement (C4) is crucial to the activation of the classical complement pathway, a key defense against invading microorganisms. The two isotypes of C4, C4A and C4B, have very different in vitro activities. An increased incidence of total C4B deficiency was found in white patients with Streptococcus pneumoniae, Haemophilus influenzae, or Neisseria meningitidis infection (14% of bacteremic children vs. 2% of race-matched controls, P = .02). In black patients, however, there was no difference in incidence of C4B deficiency between bacteremic patients and race-matched controls (7% and 5%, respectively, P greater than .5). These data suggest that, at least in whites, total C4B deficiency is a risk factor for invasive disease with these three encapsulated organisms.