Antiglobulin Testing for CR1‐Related (Knops/McCoy/Swain‐Langley/York) Blood Group Antigens: Negative and Weak Reactions Are Caused by Variable Expression of CR1

The Knops, McCoy, Swain‐Langley and York antigens have recently been identified as being on complement receptor type 1 (CR1, CD35, C3b/C4b receptor). We examined the relationship between CR1 expression and the reactivity of the CR1‐related blood group antigens with their specific antibodies. RBC from donors of selected phenotypes were tested by hemagglutination using two monoclonal antibodies to CR1, as well as anti‐Kn^a, ‐McC^a, ‐S1^a, ‐‘Kn/McC’ and ‐Yk^a. Monoclonal antibodies 3D9 and Ell required ˜250 and ˜400 CR1/RBC to obtain a positive reaction. Agglutination of antigen‐positive cells by human polyclonal antisera was related to the CR1/RBC: thus, cells expressing 20–100 CR1/RBC were negative and included the previously designated null phenotypes for this collection, 100–150 were weak or negative, and > 200 were usually positive. One RBC sample carried Yk^a on the 190,000 dalton (A or F allele), but not the 220,000 dalton (B or S allele) variant of CR1, and gave inconsistent reactions with Yk^a antisera. These data provide an explanation for certain of the serologic characteristics of the CRl‐related blood group antigen system.