Allergenicity versus Sensitivity
Dr Hugh Sampson is professor of pediatrics and head of the Jaffe Food Allergy Institute at Mount Sinai Medical Centre in New York. He explains the difference between being ?sensitised? to a food and having an allergic reaction to it - not at all the same thing.
When assessing whether a patient is likely to suffer from an allergy (or other allergies apart from those already known about) an allergist will frequently test for foods that a patient has never knowingly eaten, or is eating without obvious problems.
The likelihood of multiple allergies
Around 35% of infants with milk or egg allergy do go on to develop other food allergies while approximately 33% of children with peanut allergy will develop allergy to at least one tree nut.
Negative tests are therefore very reassuring as they suggest that the specific food tested can be eaten safely. (Of course this does not apply in allergic disorders which are not due to IgE antibodies, such as those whose symptoms are limited to the gut.)
Positive test - but no reaction
The problem arises when a patient (adult or child) tests positive to a food that he or she is eating with no apparent problems.
Although no diagnostic test in medicine is 100% accurate, the discrepancy between a positive laboratory test for a food and the absence of clinical symptoms (eg a reaction) is especially disconcerting when dealing with food allergy. Much of this confusion stems from a misinterpretation, or ‘over-interpretation’ of what the test can actually do.
When determining whether someone is likely to react to a food, the allergist must weigh a number of factors, including a detailed history, skin prick test results and food-specific IgE antibody levels. Even after considering all these factors, the allergist may still not know whether a patient will react to a specific food, and a food challenge may be recommended.
Sensitivity to untried foods or foods which do not cause a reaction
Many parents wonder how their children can have positive skin tests or blood tests to foods that they have never eaten, since you cannot make IgE antibodies against something that your immune system has never met. Because many foods are made up of related proteins (ie botanically related, such as legumes: peanuts, peas, green beans, lentils etc) the skin or blood test may not fully discriminate between various members of food families. Consequently tests to a food botanically related to an allergen to which the patient is known to react may be positive, even though the patient will not react to that food when they eat it.
This tendency for IgE antibodies to ‘bind’ to several different related foods is called ‘Cross-reactivity’.
About 90% of milk- or egg-allergic patients can eat beef or chicken, respectively, even though their skin tests are frequently
positive to both.
Other routes of sensitisation
In addition it is possible that infants become exposed to food proteins from inevitable and unsuspected places in the environment.
Researchers have suggested a number of possibilities: processed food contaminated with other foods, inhalation of food protein in vapour particles from cooking or in house dust, small amounts of food proteins passed in breast milk, residual food on parents’ or siblings’ hands contacting the skin of babies with eczema. And possibly by contact with food protein in the mother before the child is born.
IgG antibodies are made by the immune system to help protect us from infection; however, IgG antibodies also are made against the foods we eat and are found in most individuals.
Levels of IgG antibodies specific to certain foods may be higher in some individuals who have various gastrointestinal disorders, but this does not necessarily signify an allergy to a specific food.
Over the past decade research has enabled the allergist to diagnose more accurately which patients will actually react to a food and when they are likely to have ‘outgrown’ their food allergy, but in many cases, the physician-supervised food challenge is still necessary to provide the patient with a reliable diagnosis.
Courtesy of Food Allergy News, the newsletter of the American Food Allergy and Anaphylaxis Network (FAAN). More information from (001) 800 929 4040 www.foodallergy.org
First published in 2006
• If this article was of interest you will find many other articles on unlikely allergies and allergy connections here – and links to many relevant research studies here.