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, BFF1 and BFS1, 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 BFFM and BFSM, were found to exist in these populations. The BFFM allele, which was found only in Caucasians, codes for a functionally inactive factor-B product, whereas the BFSM 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 BFF1 and HLA-B18 and BFS1 and HLA-Bw50 but an even stronger association between these BF alleles and alleles of the two C4 loci. BFF1 occurred most frequently on a C4A3,BQ0 haplotype, whereas the BFS1 allele was usually found on a C4A2,B1/BQ0 haplotype. HLA haplotypes carrying the BFFM and BFSM alleles all carried the more common C4A3,B*1 haplotype.