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. ||||| A boy in Canada mysteriously became allergic to fish and nuts after he received a blood transfusion, according to a new case report. 
 
 The 8-year-old boy had no history of being allergic to any foods, and was undergoing treatment for medulloblastoma, a type of brain cancer. A few weeks after receiving a blood transfusion, he experienced a severe allergic reaction called anaphylaxis within 10 minutes of eating salmon, according to the report, published online today (April 7) in the Canadian Medical Association Journal. 
 
 His doctors suspected that the blood transfusion had triggered the reaction, they wrote in the report. After treating the patient with a drug containing antihistamines, the doctors advised him to avoid fish and to carry an epinephrine injector in case he had another reaction. [9 Weirdest Allergies] 
 
 But four days later, the boy was back in the emergency department after eating a chocolate peanut butter cup. Blood tests and a skin prick test suggested that he was allergic — at least temporarily — to peanuts and salmon, so his doctors advised him to avoid nuts and fish. 
 
 "It's very rare to have an allergic reaction to a previously tolerated food," said the report's senior author, Dr. Julia Upton, a specialist in clinical immunology and allergy at the Hospital for Sick Children in Toronto. "The overall idea is that he wasn't allergic to these foods," but in the blood transfusion, he received the protein that triggers an allergic reaction to them, she said. 
 
 That protein, called immunoglobulin E, is an antibody associated with food allergies, Upton said. When it encounters a specific allergen, it causes immune cells to release chemicals such as histamine that lead to an allergic reaction. 
 
 A skin-prick test given to the 8-year-old child showing that he has acquired reactions to peanut, tree nut mix, fish mix and salmon. Credit: Reproduced with permission from the Canadian Medical Association. 
 
 However, because the boy's body itself did not make such antibodies against fish and nuts, his doctors said they suspected his allergies would go away within a few months. 
 
 Acquiring allergies from a blood donor is rare, but not without precedent. The researchers found two other case reports, both in adults, in which patients acquired temporary allergies from blood plasma. In a 2007 case, an 80-year-old woman had an anaphylactic reaction to peanuts. An investigation showed that her 19-year-old plasma donor had a peanut allergy, according to the report in the journal Archives of Internal Medicine. 
 
 In the new case, the 8-year-old also received plasma, the liquid part of blood that contains antibodies. The researchers inquired about the donor to Canadian Blood Services, and found that the donor did have an allergy to nuts, fish and shellfish. The service did not have any more blood from the donor, and subsequently excluded the individual from making future donations, the researchers said. 
 
 About five months later, blood tests showed that the boy's immunoglobulin E levels to salmon and peanut were undetectable. By six months, his parents had gradually and successfully reintroduced nuts and fish back into their son's diet. 
 
 However, Upton said, "In general, we would recommend that this be done under medical supervision," just in case there is a medical emergency. 
 
 It's unclear how doctors could prevent future cases, she said. Neither Canadian nor American blood service organizations bar people with allergies from donating blood. And testing donated blood for levels of immunoglobulin E doesn't always predict allergies. Some people with high levels of immunoglobulin E don't have allergies, and others with low levels of the protein do, she said. 
 
 "Clearly, the safety of the [blood] supply is of everyone's utmost concern," but more research is needed to determine how best to avoid the transfer of allergies, and how frequently this happens, Upton said. 
 
 "I think it's hard to make sweeping recommendations based on one case report," Upton said. 
 
 In the United States, "If a donor is feeling well and healthy on the day of donation, they are typically eligible to donate," said Dr. Courtney Hopkins, the acting chief medical officer for the east division of the American Red Cross. "We will defer donors on the day of donation if they are not feeling well and healthy, if they have a fever, or if we notice they have problems breathing through their mouth." 
 
 Donors can learn more about blood-donation eligibility here. Individuals with allergies shouldn't be dissuaded from donating, Hopkins added. 
 
 "We always need blood. We always need blood donors," Hopkins told Live Science. 
 
 Follow Laura Geggel on Twitter @LauraGeggel. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science. ||||| The preliminary findings from ongoing research presented at the American Thoracic Society 2015 International Conference suggests that many children who have asthma also have peanut allergies but do not know it. 
 
 
 
 Of the children with asthma who had been tested for IgE, about 22% tested positively for peanut sensitivity. Of the children with asthma who had been tested for IgE, about 22% tested positively for peanut sensitivity. 
 
 Up to 3% of children in the US, Western Europe and Australia are affected by peanut allergies, with evidence also emerging that this sensitivity is beginning to affect children in Asia and Africa. These allergies develop early in life and are rarely outgrown. 
 
 Exposure to peanuts among people who have this allergy can sometimes result in a severe and potentially fatal reaction called anaphylactic shock. Because of the seriousness of anaphylactic shock, it is important that children with this sensitivity and their families are constantly vigilant about what foods they are exposed to. 
 
 The children considered to be most at risk are those with a family history of peanut allergy, have an allergy to eggs or have eczema. However, there is also an association between peanut allergies and childhood asthma. 
 
 Lead author of the new study, Dr. Robert Cohn, explains that "many of the respiratory symptoms of peanut allergy can mirror those of an asthma attack, and vice versa." These symptoms include shortness of breath, wheezing and coughing. 
 
 Dr. Cohn and colleagues examined the charts of 1,517 children from the pediatric pulmonary clinic at Mercy Children's Hospital in Toledo, Ohio. The researchers looked for documentation of peanut allergies and whether blood tests had established if children have the antibodies that indicate a potential reaction to peanuts, known as IgE. 
 
 About 11% of the charts contained documentation of a history of peanut allergy, and nearly 44% of the children underwent IgE testing for peanut allergy at some point. Of the children who had been tested, about 22% tested positively for peanut sensitivity. 
 
 Despite this, 53% of those who tested positively and their families had no knowledge of this sensitivity to peanuts. 
 
 The researchers also found that even though the prevalence of testing positively varied across age groups, the prevalence of whether the peanut allergy was known was "strikingly similar" across all age groups. 
 
 Children with asthma should undergo peanut sensitivity tests 
 
 Dr. Cohn believes that children with asthma may benefit from peanut sensitivity screening: 
 
 "This study demonstrates children with asthma might benefit from a test for peanut sensitivity, especially when control of wheezing and coughing is difficult to achieve. If a physician is having this problem, or if a parent notices it in his or her asthmatic child, they should consider testing, even if they believe their child is not sensitive to peanuts." 
 
 Dr. Cohn concludes that investigation should continue to discover more about the relationship between peanut allergies and asthma in children. 
 
 In February, Medical News Today reported on research claiming that if babies eat peanut products regularly and frequently before the age of 11 months, even those at high risk of a peanut allergy may not develop it. 
 
 Less than 1% of children in that study who were assigned to a group given peanut products three times a week or more had developed peanut allergies by the age of 5. By comparison, 17.3% of the children in a group who were told to avoid peanut-containing foods had developed peanut allergies by the age of 5.
– 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.)