C4B3 allotype with a novel Ch phenotype.

The fourth component of complement (C4) has two classes of protein, C4A and C4B, both of which have many allelic forms. The serological determinants Rodgers (Rg1, Rg2) and Chido (Ch1, Ch2, Ch3) are generally associated with C4A and C4B, respectively. The C4B3 allotype has been detected in a single Canadian family that expresses a novel Ch phenotype, Ch:-1, 2, -3. There was no information for the Rg determinants, as the C4A2B3 haplotype would normally express Rg on the C4A protein. Other C4B3 allotypes in informative families have different Ch phenotypes, and the relationships of these within extended major histocompatibility complex haplotypes are discussed in this paper.