The BF locus and HLA: rare alleles coding for functionally active and inactive factor-B products.

The genetic polymorphism of properdin factor B (BF) was studied in different populations. The rarer alleles, BF*F1 and BF*S1, occurred in Caucasians, were less frequent in North American blacks, and were not demonstrated in any of three Oriental populations studied. Two further alleles of BF, termed BF*FM and BF*SM, were found to exist in these populations. The BF*FM allele, which was found only in Caucasians, codes for a functionally inactive factor-B product, whereas the BF*SM allele (found in a single Chinese individual), like other alleles of BF, codes for a functionally active product. HLA haplotype analyses in individuals carrying the rarer alleles of BF revealed not only a strong association between BF*F1 and HLA-B18 and BF*S1 and HLA-Bw50 but an even stronger association between these BF alleles and alleles of the two C4 loci. BF*F1 occurred most frequently on a C4A*3,B*Q0 haplotype, whereas the BF*S1 allele was usually found on a C4A*2,B*1/B*Q0 haplotype. HLA haplotypes carrying the BF*FM and BF*SM alleles all carried the more common C*4A3,B*1 haplotype.