Food allergies: the basics

Abstract

IgE-associated food allergy affects approximately 3% of the population and has severe effects on the daily life of patients-manifestations occur not only in the gastrointestinal tract but also affect other organ systems. Birth cohort studies have shown that allergic sensitization to food allergens develops early in childhood. Mechanisms of pathogenesis include cross-linking of mast cell- and basophil-bound IgE and immediate release of inflammatory mediators, as well as late-phase and chronic allergic inflammation, resulting from T-cell, basophil, and eosinophil activation. Researchers have begun to characterize the molecular features of food allergens and have developed chip-based assays for multiple allergens. These have provided information about cross-reactivity among different sources of food allergens, identified disease-causing food allergens, and helped us to estimate the severity and types of allergic reactions in patients. Importantly, learning about the structure of disease-causing food allergens has allowed researchers to engineer synthetic and recombinant vaccines.

Keywords: Allergen; Diagnosis; IgE; IgE-Associated Food Allergy; Immunotherapy; Multiallergen Test; Therapy.

Rudolf Valenta

Heidrun Hochwallner

Birgit Linhart

Sandra Pahr

Figure 1

Classification of food intolerance. Adverse reactions to food can be classified as toxic or nontoxic reactions. Nontoxic reactions are categorized further as immune-mediated or non–immune-mediated. The most common adverse reactions are based on non–immune-mediated mechanisms such as enzyme defects as observed in lactose intolerance. Hypersensitivities involving the adaptive immune system can be subdivided into 4 categories (types I–IV). Type I reactions are always associated with the formation of IgE against food allergens and therefore can be called IgE-associated food allergies. There is firm evidence for an involvement of IgG in type II or type III reactions in immune-mediated adverse reactions to food, whereas type IV reactions, which involve T cells, have important roles in disorders such as celiac disease. There is evidence that the innate immune system, which includes complement, Toll-like receptors, and innate immune cells, also mediates immune reactions against certain food components.

Figure 2

Time course, pathogenesis, and manifestations of food allergies. IgE-associated food allergies appear to develop early in childhood. This process is termed allergic sensitization. (A) Allergen contact via the gastrointestinal tract, via the respiratory tract, and eventually via the skin induces IgE production (primary sensitization) in genetically predisposed individuals. Repeated allergen contact activates allergen-specific T cells and induces IgE responses during the secondary immune response. Factors that affect the epithelial barrier (red arrows) and the extent to which allergens are digested or degraded are important for primary sensitization and boosting of secondary immune responses. SIgA and T-regulatory cells may be important for exclusion of allergens from the intestinal lumen and induction of tolerance, respectively. (B) The balance between allergen-specific IgE and blocking IgG helps determine whether or not a patient will develop symptoms. Allergen avoidance could reduce levels of allergen-specific IgE to below the threshold for symptom induction (lower panel), whereas exposure could increase production of IgE, leading to symptoms (upper panel). If allergen exposure induces allergen-specific IgG, which blocks the interaction between the allergen and IgE, then symptoms might be reduced (middle panel). (C) Allergy symptoms are caused by repeated contact with the oral allergen, via the immediate allergic reaction (allergen-induced cross-linking of mast cell–bound IgE by allergen and then activation of allergen-specific T cells), and then by other inflammatory cells, such as eosinophils and basophils, during late-phase and chronic inflammation. Factors that affect the epithelial barrier and the extent of allergen degradation affect the amount of allergen intrusion and the magnitude and type of inflammation. After allergen ingestion, inflammation develops not only in the intestine, but in other organs, such as the skin, respiratory tract, and circulatory system (right). These allergens and allergen fragments are internalized and distributed throughout the body (left). MHC, major histocompatibility complex; T-reg, T-regulatory cell; TCR, T-cell receptor.

Figure 3

Diagnosis and management of IgE-associated food allergies. The diagnosis of a food allergy involves a case history and a demonstration of allergen-specific IgE production. Provocation tests and diets are used to identify disease-causing allergens. Based on this information, allergen-specific forms of treatment can be selected. This approach currently is being introduced into clinical practice. CFSE, carboxyfluorescein succinimidyl ester; ELISA, enzyme-linked immunosorbent assay; RBL, rat basophilic leukemia.

Figure 4

Component-resolved diagnosis of a food allergy. Different sources of food allergens contain several allergenic molecules (components); these can be produced as recombinant proteins or purified from natural sources. These allergens can be classified into IgE-reactive components (green), which are poor activators of inflammatory cells and therefore induce little or no clinical reactions; components that induce mild or mainly local symptoms (yellow); and components that often are associated with severe and systemic allergic reactions (red). Microarray technology can be used to determine reactivity profiles of patients. This process can be used to identify individual allergens that cause disease and foods to which patients are most likely to respond. The severity of reactions also can be predicted.

Figure 5

Allergen-specific forms of prophylaxis and treatment. Multiplex allergen systems can be used to identify disease-relevant food allergens in populations. (A) Based on the mapping of antigen epitopes recognized by patients’ IgE and T cells, 4 molecular approaches are being developed for prophylaxis and treatment. These are as follows: recombinant wild-type allergens, carrier-bound B-cell epitope–containing peptides (which do not react with IgE, have reduced allergen-specific epitopes recognized by T cells, and induce allergen-specific IgG), recombinant hypoallergens (which have reduced reactivity with IgE and fewer epitopes that interact with T cells, and induce allergen-specific IgG), and peptide epitopes that interact with T cells (but do not react with IgE or induce allergen-specific IgG). (B) Allergen-specific treatment can be prophylactic (prenatal or early postnatal) or be given after sensitization has taken place (specific immunotherapy). Active vaccination, passive immunization with allergen-specific antibodies, and tolerance induction are options.