Likelihood of a serious ABO HTR, shown as a pyramid whose base represents the probability of events predisposing to incorrect blood component transfusion, whose successive layers show the likelihood of increasingly more hazardous (as well as less likely) events sometimes leading to mortality from ABO HTR, and whose tip represents mortality. The likelihoods indicated are based on data reported by surveillance systems operating in several countries^1-6  and are expressed per number of red blood cell (RBC) units transfused.