According to a letter published this week in the New England Journal of Medicine, read about it here, a young boy displayed symptoms of anaphylaxis after a blood transfusion. The patient in this case has a severe allergy to peanuts, and the blood donors in this case had been eating peanuts within a few hours of donating blood. The patient began symptoms of anaphylaxis during the transfusion, and responded to epinephrine.
The doctors hypothesized that the reaction was due to the donors' consumption of peanuts, and that other similar reactions have occurred, but went unreported due to the lack of an explanation.
And all I can think is, "Really? Another risk with food allergies?"