The origin of a food allergy usually remains a mystery. Not so for an eight-year-old boy who received a blood transfusion unexpectedly brimming with antibodies against salmon and peanuts—two foods he had routinely consumed in the past. A few weeks after receiving transfusions, when he had a serious allergic reaction within 10 minutes of eating salmon and another after he ate a chocolate peanut butter cup, his doctors soon identified the source of the problem. Although transfusion-borne allergies are not unheard of, they are extremely rare.
 
“Allergies are so common in the population so we would anticipate that the rate of such events might be higher, and yet they have only been documented a few times in the literature,” says Julia Upton, an allergist at the Hospital for Sick Children in Toronto who wrote about this incident in the Canadian Medical Association Journal. In prior donor-linked allergy cases, a patient developed new allergies to foods, drugs or other allergens like grass following a transfusion from a donor who harbored such allergies. Fortunately, each time the allergies were short-lived and dissipated after several months because the patients did not produce the allergen antibodies themselves. With this patient, too, the allergies faded over several months.
 
“We’ve had two such cases reported to Canadian Blood Services in the past decade, and we distribute over a million blood components for transfusion every year,” says Robert Skeate, Canadian Blood Services’ associate medical director for eastern Canada. But even though the transference of allergen antibodies via transfusion is apparently rare, the principle behind it makes sense. Clinicians purposefully transfer antibodies to give patients protection against infections, so it is not surprising that other antibodies could be transferred and cause ripple effects, Upton says. Large amounts of immunoglobulin-E (IgE) antibodies remain in blood products even after storage of more than a month. Typically, fresh frozen plasma will contain the largest amount of the antibodies, followed by platelets and then red cells because all three blood components contain plasma, which can contain antibodies.
 
Still, multiple events must come together for a patient to have this rare allergic reaction. First, the blood donor must have high levels of IgE antibodies—those that react against allergens. Second, a substantial amount of blood product must be given to the patient. Then, in order to detect the new allergy, the patient would have to be exposed to the specific allergen the antibodies would react against within a few months of receiving the transfusion. That window is tight, because passively acquired antibodies will naturally fade after a few months and the transient allergy will disappear. IgE is estimated to have a half-life of just a few hours or days, but once it enters the body and binds to cells, it can remain detectable for weeks or months and cause allergic reactions.
 
Blood donors in the U.S. and Canada are not usually screened for allergies or asked to defer donation if they have a history of allergy. With this case, Canadian Blood Services officials traced the problematic blood product back to a donor with several allergies, checked to see if any more of that person’s blood was in the donor pool (it was not) and barred that person from making future donations. This incident provided “sufficient reason to think it may happen again in the future,” so Canadian Blood Services took this step as a precaution, Skeate says.
 
Typically, blood donors are only asked if they are currently experiencing any allergy symptoms at the time of donation (and asked not to donate if the answer is “yes”). That protocol makes sense, Upton says. Even if donors submitted to allergy blood tests, the results would not be definitive—they could pick up high levels of IgE antibodies but that person, or any recipients of their blood, may not have any actual reaction to the allergens in the real world. With that in mind, Upton and her co-authors are not calling for any changes in blood donation policy. “It would be very difficult to reduce the risk of such a rare reaction without substantial blood donor loss, and that’s one reason the policy is the way it is,” she says. Still, if doctors are on the lookout for the development of allergies after a blood transfusion, then the field will likely get a better sense of how common this effect is, she says.