"Right away, we wondered what has happening that was different," Dr. Julia Upton, the boy's allergist and the study author, told ABC News. "Why would he all the sudden react to a food that he clearly has eaten for years?"
Four days later, he had another reaction, this time to a peanut butter cup. Again his face turned red, but this time, he vomited, according to the study.
That's when the boy's oncologists called in Upton, a clinical immunology specialist at The Hospital for Sick Children in Toronto, Ontario. She knew about "passive transfer" of allergies from blood products and reported it to the hospital transfusion team, who contacted the Canadian blood bank, and learned a donor had several food allergies.
"It just so happened that one of the blood products that the child received contained a lot of plasma from that donor," Upton said. "The plasma is where the antibodies are."
Antibodies fight off viruses and harmful bacteria, but in people with allergies, they act up and cause problems when presented with things like peanuts and salmon.
But unlike a person who is truly allergic to a food, the boy wasn't making the antibodies on his own. They were simply in the plasma he'd received. As a result, they went away within a few months and he could eat fish and nuts again, Upton said, stressing that this is very rare.
Dr. Stacy Dorris, an allergist at Vanderbilt University Medical Center who was not involved with the boy's treatment, said blood products are often pooled, meaning several donors contribute to each bag.
“There had to be a high enough concentration from a single donor” to produce enough antibodies to cause these reactions, she said.
“I think it’s exceedingly rare,” Dorris said.
Upton said the boy has since been doing well.
"He eats everything now," Upton said. "The last time I spoke to him with the family about this case was around 6 months ago, and his mom told me everything was great."