Number of alloantibodies as a function of transfusion count for patients with at least one alloantibody at an adult study hospital: 4452 patients, 6302 alloantibodies, and 28 106 transfusions. Circles represent the mean number of antibodies for patients binned by transfusion count. The black lines are centered in each bin and extend up and down by 2 sample SDs from the mean. There are 3 patients with more than 250 transfusions not shown. The line shows a least-squares linear fit to the data. This fit suggests that many (∼70) additional transfusions are needed for the acquisition of a single additional antibody. The dashed line shows the relationship if each transfusion had an independent effect equal to that of the first.