Rodgers (Rg) and Chido (Ch) blood groups are antigenic determinants of the fourth component of human complement (C4). Nine determinants have been defined by means of hemagglutination-inhibition (HAI) with polyspccific human antiserums. The association of C4A isotypes with Rg and of C4B isotypes with Ch is strong hut not complete. Derived amino acid sequences from the C4d region of selected C4 allotypes of known antigenic expression have provided support for the previously reported complex serologic interrelationships. A structural model for antigenic determinants at four polymorphic sites, incorporating sequential and conformational epitopes, was subsequently prpposed. Allotype and Rg/Ch data obtained from donors and patients, many with accompanying families, have augmented the model and revealed no exceptions. The antigenic determinants, therefore, make an important contcihution to the complex polymorphism of C4.The Chido (Ch) and Rodgers (Rg) blood groups were first detected in 1967 sup(1) and 1976 sup(2) by antiserums from patients who had been transfused. They differed from other blood groups in two respects: 1) they were linked to HLA on chromosome 63 sup(3, 4) and 2) their antigens could be detected on red cells and in plasma. sup(2,5) Both Ch and Rg were of high frequency ( > 90%), so an antithetical relationship was not possible; nevertheless, the rare Rgand Chphenotypes were found to be in negative association. sup(6) In 1978, O'Neill et al . sup(7) demonstrated that Ch and Rg were antigenic determinants of the fourth component of complement (C4), which had already been linked to HLA; sup(8,9) this correlation led to the conclusion that there were two closely linked C4 genes, C4A(F,) and C4B(S). sup(10) A strong association exists between the C4A isotype and Rg and the C4B isotype and Ch, but rare reverse associations of Cb on C4A sup(11) and of Rg on C4B sup(12) have also been detected. By means of hemagglutination-inhibition (HAI), immune human antiserums were found to be polyspecific sup(13) and capable of defining eight high-frequency determinants, two Rg and six Ch, and one, WH, of much lower frequency (approximately 20%) .13-15 Informative individuals showed either the lack of high-frequency antigens in homozygotes, a recessive trait, or the presence of WH, which was detected in both homo-and heterozygotes, a dominant trait. sup(13-15) The electrophoretic polymorphism of C4, with 13 C4A and 22 C4B allotypes, sup( 16) and the antigenic polymorphism have inevitably shown strong associations. 14,17
Materials and Methods
Human anti-C4The reagents are collectively termed anti-Rg and anti-Ch, but it is now known that, with few exceptions, they are polyspecific. 13-15 Anti-Rg (1,2) has been detected in Rg: -1, -2 patients with no C4A and anti-Ch (1 to 6, WH) in Ch:-1,-2,-3,-4, -5,-6, WH-patients with no C4B. There has been one report of a C4B Ch+ patient (Ch:1,-2,3,4,-5,6) with alloanti-Ch 2, 5. sup(18) A transfused C4-deficient patient had, in addition to anti-Rg and antiCh, an antibody cross-reactive ...