Restoration of complement function in vivo by plasma infusion in factor I (C3b inactivator) deficiency.

The serum complement activities of a 1-year-old infant with recurrent life-threatening bacterial infections and persistent C3-Coombs-positive red blood cells were investigated. The patient's serum had a depressed serum level of CH50, C3, factor B, and factor H as well as undetectable antigenic or functional factor I. The complement profile of the parents was normal, with the exception of factor I, which was approximately 50% of normal in each parent. The Coombs positivity of the patient's red blood cells could be reversed in vitro by incubation with normal serum containing factor I. Infusion of normal plasma into the patient resulted in increased levels of CH50, with concomitant increases in serum C3, factor B, and factor H levels. The patient's red blood cells became transiently Coombs negative. At no time after plasma infusion was factor I detectable in the patient's serum. All complement functions and the C3 Coombs reactivity of the patient's red blood cells returned to preinfusion levels within 14 days. These findings are consistent with an inherited deficiency of factor I and emphasize the critical role this protein plays in the regulation of the alternative complement pathway. Plasma therapy may be an adjunct to the management of acute infection in patients with factor I deficiency.