Warm IgM anti-IT causing autoimmune haemolytic anaemia

A patient with autoimmune haemolytic anaemia due to an autoagglutinin of anti-IT specificity is reported. The antibody was of the IgM class and bound complement; unexpectedly, it reacted optimally at 37 degrees C. A prozone was noted when testing serial dilutions of the patient's serum against appropriate red cells in saline, but was not found when testing an eluate prepared from the patient's red cells. Investigations suggested that the prozone was due to a blocking antibody of high molecular weight, which appeared to be specific for I and IT.