Abstract
Serum C2 activity was measured in 135 individuals drawn from a panel of 418 tissue-typed blood donors. The study group included all donors with HLA antigens A10 and B18. Heterozygote C2 deficiency (C2Dhet) was defined by reference to the range of C2 activity in previously studied obligate heterozygotes. Five donors were C2Dhet. Family studies confirmed that C2Dhet was associated in all instance with an A10, B18 haplotype. The minimum frequency for C2Dhet was 1.2% in the panel of 418 donors and 62.5% in those donors with an A10, B18 haplotype.