Analysis of Knops blood group antigens on CR1 (CD35) by the MAIEA test and by immunoblotting.

Kna, McCa, Sla and Yka are red cell antigens of relatively high frequency, located on complement receptor 1 (CR1, CD35). Antibodies to these Knops system antigens are not uncommon. They are not haemolytic and do not reduce the survival of transfused incompatible red cells, but they are a nuisance in transfusion laboratories as they can cause 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 murine monoclonal anti-CR1. In addition to confirming that Kna, McCa, Sla and Yka are located on CR1, the MAIEA test was used to confirm that Csa is not on CR1. Red cells of the Helgeson phenotype, the null phenotype of the Knops system by conventional serological methods, have levels of Kna, McCa, Sla and Yka intermediate between those of alpha-chymotrypsin-treated cells (which lack Knops system antigens) and those of positive control cells. Level of expression of Knops system antigens is very variable and intensity of staining of immunoblots probed with monoclonal anti-CR1 correlated with strength of Knops system antigens, as determined by the MAIEA test. In individuals heterozygous for alleles producing different allotypes, separate bands representing each allotype on an immunoblot showed identical intensity of staining, suggesting that the quantity of CR1 on red cells is controlled, at least in part, by a locus independent of CR1. An analysis of CR1 on red cells of individuals who have made Knops system antibodies suggested that the Knops system antigens and the antibodies that detect them are complex and heterogeneous.