RBC transfusion counts for patients at an adult study hospital who formed different numbers of antibodies. The histogram shows the number of patients receiving a particular number of transfusions. The median number of transfusions is shown as a circle, and the horizontal line extends from the 25th percentile to the 75th percentile. The median for the single-antibody group is 0, meaning these patients responded to transfusions received outside our study hospital or study period, or had naturally occurring alloantibodies, or acquired their antibodies through some means other than transfusion, such as pregnancy.