Although food allergy can arise to any food, the allergens responsible for more than 85% of food allergy are: milk, egg, peanut, tree nuts, shellfish, fish, wheat, sesame seed and soy [5
]. These are also the “priority” allergens defined by Health Canada. It is the protein component, not the fat or carbohydrate component, of these foods that leads to sensitization and allergy. The allergenic segments or “epitopes” of these proteins tend to be small (10 to 70 kd in size), water-soluble glycoproteins that are generally resistant to denaturation by heat or acid and, therefore, can remain intact even after processing, storage, cooking and digestion [3
]. Examples of these glycoproteins include caseins in milk, vicillins in peanut, and ovomucoid in egg. In general, allergies to additives and preservatives are uncommon.
Food-induced allergic disorders are broadly categorized into those mediated by immunoglobulin E (IgE) antibodies or by non-IgE-mediated mechanisms. IgE-mediated allergic responses are the most widely recognized form of food allergy and are characterized by the rapid onset of symptoms after ingestion. During initial “sensitization” to the food, consumption of the allergenic food protein stimulates production of IgE antibodies specific to that food which then bind to tissue basophils and mast cells. When the causal foods are subsequently eaten, they bind to their specific IgE antibodies and trigger the release of mediators, such as histamine, prostaglandins and leukotrienes, causing “clinical reactivity” and allergic symptoms. It is important to note that sensitization can be present without
clinical reactivity, meaning that specific IgE to a food is present, but no reaction occurs with exposure [2
Non-IgE-mediated (cell-mediated) food allergy is less common and results from the generation of T cells that respond directly to the protein, leading to the release of mediators that direct certain inflammatory responses (e.g., eosinophilic inflammation) and can cause a variety of subacute and chronic disease states. These types of reactions typically affect the gastrointestinal (GI) tract and skin and include: dietary-protein-induced enterocolitis and proctitis, celiac disease and its related skin disorder dermatitis herpetiformis. Celiac disease is associated with a specific immunoglobulin A (IgA)-mediated sensitivity to gluten (a protein found in wheat, barley, rye and certain other grains), and is associated with chronic inflammation and damage to the villi of the small intestine. Dermatitis herpetiformis is a chronic skin disorder that may occur simultaneously with celiac disease or alone [2
Some disorders are associated with a mixed IgE-/cell-mediated pathophysiology to food, such as atopic dermatitis, eosinophilic gastroenteritis and eosinophilic esophagitis (EoE; see articles on EoE and atopic dermatitis in this supplement). In these disorders, the association with food may not be demonstrated in all patients.
The spectrum of food-allergy-associated disorders according to pathophysiology is shown in Figure . It is important to note that food allergy is not a cause of conditions such as migraines, behavioural or developmental disorders, arthritis, seizures or inflammatory bowel disease.
Figure 1 Spectrum of food allergy disorders according to pathophysiology [3,4,7]