1. A severe hemolytic reaction was observed in an "A-intermediate" recipient following transfusion of group O blood containing A antibodies in high titer. Hemoglobinemia persisted for at least thirty hours, during which time the patient became markedly anemic. Azotemia, hyperbilirubinemia, spherocytosis and increased osmotic and mechanical fragilities of the patient’s erythrocytes were observed for nine days.

2. The recipient was rendered temporarily incompatible with prospective A1 donors as a result of incomplete absorption of the anti-A agglutinins in the transfused group O blood.

3. The A antibodies in the serum of the dangerous universal donor were shown to have the following peculiarities : (a) When normal AB serum was used as a diluent for the group O serum and as a medium for cell suspensions, the titer against A1 cells was higher than that obtained with saline as a diluent and suspension medium. (b) After "neutralization" with soluble A and B factors, the group O serum agglutinated neither A nor B cells in "saline systems," but in "serum systems" the treated O serum still agglutinated A1 cells in very high titer and A-intermediate cells in low titer. (c) A1 cells, but not A-intermediate or A2 cells, gave positive reactions with anti-human-globulin rabbit serum (indirect Coombs test) after exposure to the neutralized O serum. (d) The group O serum hemolyzed, and fixed complement with, all types of A cells but not B cells.

These peculiarities suggested that the group O donor had developed A antibodies of immune type and predominantly of A1 specificity, but the stimulus for production of these antibodies could not be ascertained.

4. It is emphasized that the type of antibody (i.e., natural or immune), as well as the titer, must be taken into account in any attempt to assess the dangers inherent in the use of universal donors, and that "conditioning" of group O blood by addition of soluble A and B factors in conventional quantities may not effectively neutralize immune A1 and B antibodies if such be present. It is pointed out, however, that further observations are needed to show whether or not neutralization of "saline" agglutinins and hemolysins by A and B factors may render dangerous bloods relatively safe despite the persistence of incomplete antibodies.

5. Possible mechanisms of prolonged erythrocyte destruction in this case are discussed.

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