Influence of age and sex on immunization. (A) Comparison based on sex and for ages 18 to 50 years (pregnancy data were not available, and some pregnancies might have occurred earlier or later than these ages). (B) Comparison between patients whose first transfusion occurred before or after the age of 5 years. Crosses show censored data, that is, each time a patient made an antibody or the last known transfusion for the patient if they made no antibodies.