Food Allergy: Soy (less allergenic than egg, milk, wheat and peanut)

Not all food are allergenic, and not all allergenic foods are equally allergenic. In other words, foods vary in clinical allergy significance. Generally speaking, all food proteins are potentially allergenic for some people. Among all foods, 8 foods accounts for 90% of food allergies: milk, wheat, peanuts, tree nuts, eggs, fish, crustacea, and soy (1).

Soy proteins tend to be less immunologically reactive than many other food proteins.

Immunology of food allergy

Type I food allergy consists of 3 steps (2,3,4,5). First there is a sensitization starting with food antigens moving across the intestinal barrier. An immature gut, or infection will worsen the process. Antibody responses are triggered (IgG and IgA) leading to an allergic response.

Food antigens activate antigen-specific B cells and helper T cells that direct B cells to differentiate into IgE-producing plasma cells. IgE is then quickly bound by high-affinity IgE receptors mainly on the surface of mast cells. Mast cells contain large amounts of histamine which is a main inducer of symptoms of allergy. Following this insult, a large number of mast cells armed with IgE antibodies is present in circulation and tissues.

After sensitization, allergen or multivalent allergen fragments are again absorbed after ingestion. IgE antibodies on mast cells bind the allergen so that the allergen cross-links at least 2 receptor-bound IgE molecules. This sends a signal causing the mast cell to release histamine and other inflammatory mediators (degranulation). Timing and magnitude of the release is defined by allergen dose and a number of poorly understood host factors. At the end, clinical symptoms ensure when histamine and other inflammatory mediators stimulate the wide variety of allergic symptoms in other cells and organs.

Soy allergy

The first allergic reactions to soy in humans were described in 1934 (2,6). Anti-soy IgE antibodies have been identified but allergen specificity patterns are variable and complex. As many as 28 soy proteins bind to IgE from soy-allergic patients (2,7). Soy is also an aeroallergen, although the pathologies and allergen reactivity profiles are different for ingestion versus inhalation, where soy hull antigens not present in soy protein isolates seem to dominate (2,8).

Among the various food allergies, soy is less allergenic than egg, milk, wheat and peanut (2). This means that a greater soy dose is necessary to cause the same reaction as the other foods, the reaction is less severe. 

The prevalence of soy allergy was only found to be 1.2% in in a cohort of 505 children (9), and 0.4 in 243 children fed soy protein formulate during the first 6 months (10). European cohorts observed a prevalence from 0-0.7% in children subject to double-blind placebo-controlled challenge (11). Soy allergy is less common than cow’s milk allergy (CMA), but 10% to 14% of patients with CMA also present with soy protein allergy (12,13,14).

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