We report a rare ‘hypomorphic’ C4 allotype detected during routine screening in controls for the Rogers:1 epitope. C4B∗15 was distinguished by having only faint staining when using polyclonal anti-C4 antibody on agarose inimunoelectrophoresis (e.g. hypomorphic), having relatively weak hemolytic activity but being strongly reactive with monoclonal antibody to Rodgers 1. TaqI restriction fragment length polymorphism (RFLP) demonstrated that C4B∗ 15 segregated with 7 kb and 5.4 kb C4 gene fragments and with the haplotype HLA-A2,C-, B50,BW6,DR7,DQ2,DR52,S07C2(1,15). The 5.4-kb fragment was more intense than the 7.0-kb fragment, suggesting duplication of the 5.4-kb fragment. This hypomorphic C4 allotype (genotype frequency = 0.0088) has diminished expression of C4 epitopes commonly recognized by polyclonal anti-C4 and may be missed by standard phenotyping methods.
