An 8-year-old boy in Canada never had any problems eating salmon or peanuts. Then, within just a few weeks of getting a blood transfusion as part of his treatment for brain cancer, he ate salmon and experienced a severe allergic reaction within 10 minutes. The same thing happened when he ate a chocolate peanut butter cup four days later. Doctors determined he was suffering from transfusion-borne allergies, and wrote about it in the Canadian Medical Association Journal. A piece in Scientific American calls the case "extremely rare." But with allergies so common, why is that, well, the case? Such a reaction can only occur if three factors are in place, explains Scientific American. The donor must first have high levels of the type of immunoglobulin-E (IgE) antibodies that set off allergic reactions.
The patient must then receive a significant amount of that blood and, within a few months, be exposed to the allergen the IgE antibodies would react to. That's because—in good news for this 8-year-old and others like him—the IgE antibodies eventually disappear from the blood because the recipients don't produce the same antibodies on their own. In this case, the boy was again eating fish and peanut butter six months later. Scientific American points out that antibody-transfer is usually a good thing, as it helps guard against infection. And avoiding a situation like this is tough, LiveScience reports. Having allergies doesn't disqualify would-be donors (in either the US or Canada), and having a high level of IgE in the blood doesn't always correlate with experiencing allergies. (Last year, a toddler almost died from a nearly unheard-of allergy.)