SummaryErythrocytes (E) lacking high incidence blood group antigens were screened by an antiglobulin test with a monoclonal antibody to human complement receptor type 1 (CR1 ; C3b/C4b receptor; CD35) . Some examples of E lacking Knops, McCoy, Swain-Langley, and York antigens, a serologically related group, were not agglutinated . Moreover, E of the null phenotype for these same antigens were nonreactive. To further explore this relationship, E expressing these antigens were surface labeled, solubilized, and incubated with the corresponding blood group-specific antisera. CR1 was immunoprecipitated, indicating that the epitopes recognized by each of these antisera are expressed on 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-Kna, -McCa, -S1a, -'Kn/McC' and -Yka. Monoclonal antibodies 3D9 and E11 required approximately 250 and approximately 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 greater than 200 were usually positive. One RBC sample carried Yka 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 Yka antisera. These data provide an explanation for certain of the serologic characteristics of the CR1-related blood group antigen system.

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