A case of naturally occurring anti-E mimicking anti-A1

Rhesus (Rh) antibodies are usually of the acquired IgG type. Naturally occurring IgM type of Rh antibody is unusual. We report a case of naturally occurring IgM anti-E which mimicked anti-A1. ABO grouping of a 61-year-old man posted for surgery showed discrepancy. Cell group corresponded to A positive while reverse grouping showed weak agglutination with A cells. Nonreactivity of red cells with A1 lectin suggested the possibility of A subgroup with anti-A1. Anti-A1 was ruled out on further testing of serum with A1-positive red cells. Antibody screening and identification revealed the presence of anti-E. The presence of Rh E antigen in the reagent A cell was the reason for weak agglutination. Antibody titer of 4 and 2 was observed at 4°C and 37°C incubation, respectively, in the untreated serum. Antibody was undetectable in the dithiothreitol-treated serum both at 4°C and 37°C confirming the IgM nature of the antibody.