Percentage of all transfused patients at the adult study hospital who are alloimmunized as a function of transfusion count. Circles represent the proportion of transfused patients who have developed at least one alloantibody binned by transfusion count. The first bin includes all patients transfused with 0 to 10 units, the second bin includes all patients transfused with 11 to 20 units, etc. The black lines are centered in each bin and extend up and down by 2 sample SDs from the mean. The solid line shows a least-squares linear fit to the data. This fit suggests that many (∼190) additional transfusions are needed before 50% of patients will form an alloantibody. The dashed line shows the relationship if each transfusion had an independent effect equal to that of the first, in which case approximately 17 transfusions would be required to cause 50% of patients to form alloantibodies.