A 24-year-old woman who had previously been uneventfully transfused,receives a blood transfusion during surgery and sholy thereafter develops itching, generalized uicaria, laryngeal edema, and dyspnea with wheezing respiration. She has a past history of recurrent upper respiratory tract infections and frequent episodes of diarrhea. Laboratory studies are most likely to reveal decreased concentrations of which of the following immunoglobulins?