In a very rare case, an eight-year-old boy has developed an allergy to fish and nuts—foods that he used to be fine with—after receiving a blood transfusion during treatment for a type of brain cancer. The case study was published in the Canadian Medical Association Journal this week.

Weeks after the transfusion, the boy began experiencing severe anaphylaxis within minutes of eating salmon, and then again after eating a chocolate peanut butter cup, Live Science reports. Blood and skin prick tests revealed that he was (at least temporarily) allergic to nuts and fish. 

When people with food allergies donate blood products like plasma or platelets, they might end up transferring a protein called immunoglobulin E (IgE), an antibody that reacts against allergens. This can stay in the blood products even after a month of storage, and when it meets up with specific allergens, it causes the body’s immune cells to release histamine and similar chemicals to stir up an allergic reaction. 

But people with allergies aren’t really screened or barred from donating blood in Canada or in the U.S. For one thing, multiple events must come together for a recipient to develop a new allergy this way, Scientific American explains: The donor must have high levels of IgE antibodies, a substantial amount of blood product must be received, and the recipient must be exposed to the specific allergen the antibodies react against within a tight window frame of a few months.

"It is very unusual to identify someone who experienced passive transfer of allergy from blood products," says Julia Upton of the Hospital for Sick Children (known as SickKids) in a news release. While rare, it can still result in anaphylactic reactions to foods that were happily consumed (or at least tolerated) without triggering any reactions in the past. The symptoms—which can range from sudden fatigue to throat discomfort and facial swelling—should be treated immediately. 

Upton adds: "Importantly, this condition has an excellent prognosis and typically resolves within a few months." Blood tests revealed how the boy's IgE levels to salmon and peanut were undetectable about five months later, Live Science reports. And by the half-year mark, his parents were able to gradually reintroduced nuts and fish back into his diet.