Antigenic sites of Bg^a specificity have been demonstrated on leucocytes as well as on red cells. The Bg^a group shows almost complete concordance with the histocompatibility group HL-A7. Red cells show much less activity than leucocytes and did not absorb cytotoxic antibodies but leucocytes removed all haemagglutinating activity. RNAP reactions can be demonstrated by both haemagglutination and cytotoxic tests. In some instances multispecific cytotoxic antibodies may show a monospecific haemagglutinating component. Family studies showed that the HL-A groups may influence the Bg^a reaction of the red cells.

Following an attack of infectious mononucleosis, the red cells of HLA B7 patients may show a greatly increased reactivity with anti-Bg^a antibodies. This occurs from about the 3rd week of the disease, the reactivity then slowly decreasing over a period of months or years. Both specific and nonspecific reactions to other HLA antisera may also occur. Five patients were followed in detail and the reason for the increase in antigen strength was investigated without a conclusive result.