Increased activation of the alternative complement pathway in sickle cell disease.

Complement proteins play an important role in host defenses against Streptococcus pneumoniae, a major cause of serious infections in sickle cell (SS) disease. Previous studies have suggested abnormalities of the alternative complement pathway in SS disease. We measured activation of the alternative pathway in sera from patients with SS disease utilizing an enzyme immunoassay which detects C3b,P complexes, derivative of the C3b,Bb,P alternative pathway convertase. In all, 89% of SS sera had elevated concentrations of C3b,P complexes, indicative of increased alternative pathway activation. Chronic activation of the alternative pathway may contribute to impaired host defense in SS patients.