Individuals with homozygous Sickle Cell Disease (SCD) are prone to bacterial infection. SCD sera have previously been shown by others to have a defective capacity to opsonify pneumococci—this has been attributed to impaired alternative pathway function, which was postulated to be due to defective synthesis of factor B in such cases.

Sera from 100 cases of homozygous SCD have now been studied. There was reduced alternative pathway activity in one-third of sera (using a haemolytic plate assay dependent on Mg++ and agarose to activate the alternative pathway), and haemolytic factor B activity was significantly lower in this group than in the other sera. C3 concentrations were also reduced in one-third of sera.

The metabolism of purified radiolabeled factor B and C3 was studied in 12 cases to determine whether diminished synthesis or increased catabolism of these proteins occurred in SCD. Synthesis of B and C3 was normal or increased in all cases.

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