Discrepant in vitro versus in vivo interaction of M-positive donor red cells with IgG1 anti-M.

Following massive transfusion and a suspected sepsis, a patient produced a potent, high titer IgG1 anti-M alloantibody reactive at 37 degrees C with an indirect antiglobulin test [IAT] titer using anti-IgG of 1:512 versus MM red blood cells (RBC). During pretransfusion tests, the antibody was strongly reactive with M+ RBC using IAT, however, transfused M+ RBC remained in circulation and demonstrated normal red cell survival. The direct antiglobulin test remained negative and no antibody activity was detectable in eluates prepared at various times from red cells obtained from the patient post transfusion. There was no direct agglutination of red cells obtained from the patient post transfusion with autologous serum, even at 4 degrees C. However, these samples contained donor M+ RBC as determined by tests using various examples of human and rabbit anti-M antisera. The extraordinary findings of transfused M+ RBC, highly incompatible pretransfusion using IAT, coexisting with potent warm reactive anti-M antibody without evidence of in vivo antibody-antigen interaction represents an unusual discrepancy. Although the explanation for these findings remains unclear, it may indicate antigen modification of donor red cells in vivo.