A 27-year-old male patient (blood group O) arrives at the emergency room with a massive intestinal bleed (hematochezia) Within hours he has lost half of his blood volume, and you decide to transfuse. Due to human error, you transfuse blood group AB into him and within minutes he develops a fever, chills, dyspnea, and a dramatic drop in blood pressure. This reaction is most likely due to which of the following?