ANALYSIS OF KNOPS BLOOD-GROUP ANTIGENS ON CR-1 (CD35) BY THE MAIEA TEST AND BY IMMUNOBLOTTING

Kn(a), McC(a), Sl(a) and Yk(a) are red cell antigens of relatively hig h frequency, located on complement receptor 1 (CR1, CD35). Antibodies to these Knops system antigens are not uncommon. They are not haemolyt ic and do not reduce the survival of transfused incompatible red cells , but they are a nuisance in transfusion laboratories as they can caus e an incompatible crossmatch and must be identified before they can be dismissed as clinically insignificant. Human red cell alloantibodies can be shown to be Knops system antibodies by the monoclonal-antibody- specific immobilization of erythrocyte antigens (MAIEA) test, using mu rine monoclonal anti-CR1. In addition to confirming that Kn(a), McC(a) , Sl(a) and Yk(a) are located on CR1, the MAIEA test was used to confi rm that Cs-a is not on CR1. Red cells of the Helgeson phenotype, the n ull phenotype of the Knops system by conventional serological methods, have levels of Kn(a), McC(a), Sl(a) and Yk(a) intermediate between th ose of alpha-chymotrypsin-treated cells (which lack Knops system antig ens) and those of positive control cells. Level of expression of Knops system antigens is very variable and intensity of staining of immunob lots probed with monoclonal anti-CR1 correlated with strength of Knops system antigens, as determined by the MAIEA test. In individuals hete rozygous for alleles producing different allotypes, separate bands rep resenting each allotype on an immunoblot showed identical intensity of staining, suggesting that the quantity of CR1 on red cells is control led, at least in part, by a locus independent of CR1. An analysis of C R1 on red cells of individuals who have made Knops system antibodies s uggested that the Knops system antigens and the antibodies that detect them are complex and heterogeneous.