In 1,814 patients with sickle cell disease who had been transfused, the overall rate of alloimmunization to erythrocyte antigens was 18.6%. The rate of alloimmunization in this group appears to be an explicit function of the number of transfusions received because it increases exponentially with increasing numbers of transfusions. Alloimmunization usually occurred with less than 15 transfusions, although the rate of alloimmunization continued to increase when more transfusions were given. The rate of alloimmunization was less in patients with hemoglobin SC disease and sickle-beta+ thalassemia because these patients had received fewer transfusions. Children less than 10 years old had a slightly lower rate of alloimmunization than patients in other age groups even after correction for the number of transfusions given. Women were more frequently alloimmunized than men; this was largely due to the fact that women received more transfusions than men, but in the age group 16 to 20 years the increase may have been due in part to alloimmunization owing to pregnancy. Forty-five percent of those alloimmunized made antibodies of only one specificity; 17% made four or more antibodies reacting with different antigens. Antibodies to the C and E antigens of the Rh group, the Kell antigen, and the Lewis antigens were most commonly made. These findings may be important in formulating a rational transfusion policy in sickle cell disease.

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