Diagnostics and Immunotherapy in Allergic Reactions: Food Allergies

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Table of Contents

Introduction
Mechanism of Immune Response to Food Allergies
Clinical Disorders of Food Allergies
Food Allergies Diagnosis
Management and Immunotherapy of Food Allergies
Conclusions and Future Prospects
References

Introduction

Abnormal immune responses to proteins found in certain foods can cause allergic reactions when such foods are ingested by susceptible individuals. Such immune reactions are called food allergies, and the proteins which cause the reactions are called allergens. Foods such as milk, peanuts, eggs, fish, and shellfish are the ones that most commonly cause allergic reactions in the American population. Food allergy affects about 2% of children and about 8% of adults, and cause serious illnesses and in some rare cases even death.

The immune system is made up of cellular and molecular components that can induce and maintenance tolerance to allergens. Various methods have been developed for the diagnosis and management of food allergies. In this paper, the immunological processes of allergic reactions are described giving the relevant anatomical and physiological parameters. Then an overview of pathophysiological features (clinical manifestations) of food allergies is given including their traditional and current diagnostic and management (or treatment) including the immunotherapeutic techniques. Finally, conclusions are given providing a generalized picture of the scope of work still needed to solve the problems of allergies to foods and also to other environmental, domestic, and occupational allergens.

Mechanism of Immune Response to Food Allergies

Allergic reactions are usually typed 1 hypersensitivity reactions and involve two arms of the immune system, namely mast cells and immunoglobulin E (IgE). In most cases, when a person ingests food that contains an allergen, the body produces large amounts of IgE specific to food that they had previously been exposed to. The IgE that is released has a high affinity for its receptor on mast cells in the tissues and basophils in the blood, both having mediators of inflammation in their cytoplasm. This process is called mast cell sensitization. On second and subsequent encounter with the same antigen, the IgE and its receptor FcεRI are crosslinked, resulting in the synthesis and release of cytokines and lipid mediators and also the release of granules in the cytoplasm which causes the allergic reaction. Figure 1 shows the sequence of events in immediate hypersensitivity. Immune system cells like T helper 1 and T helper 2 are involved during an allergic reaction to food. The T helper 2 subset is the one that leads to IgE production through B cells activation which involves cytokines such as I-3 and IL-13.

Figure 1: Sequence of events in immediate hypersensitivity

Depending on their structures, proteins or antigens that come into contact with the surface of the intestine are taken up by different cells which can lead to varied responses. The cells that can take up the proteins or the antigens include Peyer’s patches, Microfold cells, and epithelial cells or dendritic cells. Figure 2 shows the antigen uptake sites in the intestine or the gut. After the proteins are taken up by the immune cells, they are broken up or processed into peptides which can then be presented (by antigen-presenting cells, or APCs) to be recognized by immune activation cells. The generation of peptides from intact protein antigens involves modification of the native proteins and is commonly referred to as antigen processing. On the other hand, the display of the peptides at the cell surface by the MHC molecule is referred to as antigen presentation.

As mentioned earlier, the cells that can take up the proteins or the antigens include Peyer’s patches, Microfold cells, and epithelial cells or dendritic cells. These cells are called accessory cells. Other than these accessory cells which can be found in the gut, B-lymphocytes can also internalize or process antigens and present the peptide–MHC complexes in forms that can be recognized by CD4 T cells. Generally, most mammalian cells are capable of endocytosing and processing protein antigens. However, the expression of class II MHC molecules is the critical property that enables a particular cell to function as an antigen-presenting cell (APC).

As the antigenic stimulus is activated, immune responses also decline. There are several mechanisms that can inhibit lymphocyte activation. Immunologic tolerance can be induced if an antigen is recognized by specific T lymphocytes when co-stimulatory molecules are absent, e.g. major histocompartibility (MHC) restricted molecule, or by B cells in situations where T cells help are lacking. Immunological tolerance can result from antigen-induced block on maturation and or activation of lymphocytes, or T cells anergy in certain conditions of antigenic exposure or load. Tolerogens are antigens that induce tolerance while immunogens are antigens that generate immune responses.

In review articles, it is generally stated that oral tolerance can be induced by a variety of factors (antigen or host-related) and immune system cells, the most important of which are the regulatory T cells. Food hypersensitivity can result if there are disturbances in the oral tolerance mechanism. Immune responses to foreign antigens are regulated both quantitatively and qualitatively by numerous mechanisms. Factors that influence the induction of specific immunity include the type and amount of antigen, its portal entry, and the participation of accessory cells in the immune responses. These factors may determine which functionally distinct classes of lymphocytes are stimulated, and may influence the balance between lymphocyte activation and tolerance. Figure 3 shows how immunological tolerance can be induced and Figure 4 shows immunological mechanisms of oral tolerance.

Figure 2. Sites of antigen uptake in the gut. a Antigen can be sampled by DCs that extend processes into the lumen. b, Particulate antigens are taken up by M cells overlying PPs and then delivered to DCs in the subepithelial dome region and then to underlying B-cell follicles, where IgA commitment occurs. c, Soluble antigens might cross the epithelium through transcellular or paracellular routes and then might encounter T cells or macrophages in the lamina propria or might reach the circulation

Figure 3. Induction of oral tolerance. a, When mice are immunized subcutaneously and then boosted subcutaneously with an antigen, strong in vitro cell-mediated and antibody responses to the antigen occur. b, When mice are first fed the antigen orally and then immunized subcutaneously, in vitro immune responses to the antigen are greatly reduced. c, When T cells from mice that were fed antigen are transferred to naïve mice, subcutaneous immunization of these naive mice results in reduced in vitro immune responses as well. This shows that oral feeding of an antigen can induce a T cell-mediated active inhibitory immune response

Figure 4. Mechanisms of oral tolerance. a Generation of an immune response requires ligation of the T-cell receptor with peptide-MHC complexes in the presence of appropriate costimulatory molecules (CD80 and CD86) and cytokines. b, With high doses of oral antigen, T-cell receptor cross-linking can occur in the absence of costimulation or the presence of inhibitory ligands (CD95 and CD95 ligand), leading to energy or deletion, respectively. c, Low doses of oral antigen lead to the activation of regulatory T cells, which suppress immune responses through soluble or cell surface-associated suppressive cytokines (IL-10 and TGF-b).

Clinical Disorders of Food Allergies

The clinical disorders of food allergies are generally categorized based on interrelated causes some of which can be immunological and others due to the affected organs, or body systems. Common gastrointestinal symptoms ensue from a number of food-induced allergic disorders. These systems are usually differentiated based on diagnostic tests used or sometimes on the kind of illness or syndrome the patient suffers from. Constipation, reflux, and colic are some of the symptoms of the gastrointestinal tract that are associated with allergies to food. In the majority of situations, anaphylaxis usually results from food intake in outpatients, and some reaction outcomes, e.g. after eating nuts, can be fatal especially in young patients with a history of asthma and food allergy. Fatal outcomes easily result if treatment, usually epinephrine administration, is delayed.

Food Allergies Diagnosis

A comprehensive health history combined with a thorough physical examination of the patient is a prerequisite during the clinical diagnosis of an allergy. This will include evaluation of the causative food item, the quantity ingested, allergic reaction time, and other relevant parameters including exercise by the patient, aspirin or alcohol intake, and the reaction consistency. In the majority of cases, food infrequently ingested is usually the cause of an acute allergic reaction than food that was previously tolerated, and acute symptoms of an allergic reaction can include urticaria occurring immediately after ingestion of a food item. In most cases, a food allergy may not be the causative factor for chronic symptoms such as asthma and urticaria.

Confirmation of a diagnosis can in some cases require invasive testing. However, in most cases, the diagnosis relies on the elimination of diet tests, oral food challenge responses, and food-specific IgE antibody determination. Skin prick tests (SPTs) can be routinely used to provide a rapid method of detecting sensitization for IgE-mediated disorders. However it is not always that a suspected food item will be proved to be the causative agent of the allergy after a positive test response. IgE-mediated allergic response can be appropriately confirmed by negative SPT responses. Clinical history and the pathophysiology of the disease are usually of great importance when making maximum use of the results obtained from tests in allergy cases.

Serum tests can also provide an alternative modality for evaluating IgE-mediated food allergy and help through IgE antibodies determination, and it has been found that clinical allergic reactions do correlate with higher IgE values from the diagnostic assays. It will be important to determine the specific IgE-binding epitopes on allergens because this can increase their diagnostic utility if they can be made commercially available, and more studies are needed in this area to develop better diagnostic methods. But the classical placebo-controlled oral food challenge is still the gold standard test for food allergies in patients.

Management and Immunotherapy of Food Allergies

Treatment for food allergies involves the reduction of inflammation, inhibition of mast cell degranulation, or antagonizing the effects of mast cell mediators. The introduction of small doses of the allergen into the body has been one method that is used in the treatment of food allergies. The allergen is introduced into the body in small but increasing dosages and is thought to inhibit IgE production or increase the production of other Ig subclasses. It has also been suggested that other endogenous, environmental, or dietary factors that promote sensitization of these pathways should be considered as some of them, e.g., dried spices can cause allergies and be an occupational health hazard.

In most cases, childhood allergies do naturally resolve which means that more evaluations will be needed sometimes at an increased cost to the patients. Medications that can ameliorate certain aspects of food-induced allergies already exists and classically, antihistamines have been used for such therapies to help manage symptoms of oral allergy syndromes and even to IgE-mediated skin reactions.

A number of new approaches of immunotherapy including the use of peptides on T cell epitopes that lack IgE-binding activity are being tested for use in treatment regimens of food allergies which could result in safe and specific modes of immunotherapy,which require modern characterization methods of proteins and potential allergens. Molecular therapeutic approaches are already being explored for example use of IFN-gamma encoded by cDNA, or vaccination, or IL-4 employing CpG motifs expressed by DNA animal studies. Such novel immunotherapeutic strategies are promising as effective treatment or management of allergen-induced asthma. Already, some specific genes, together with their polymorphisms, are already being studied for their potential usefulness in atopy-linked inflammatory cells action. However, genetic factors which could be important in remodeling at mucosal surfaces and in tissue repair should be understood first.

Immunotherapy will particularly be important in allergy syndromes that are persistent in patients, and also in those susceptible to food allergies. Another potential mechanism of immunotherapy that has been tested involves the injection of food allergens. However, this method has been found to be unsafe. Another alternative to antigen injection as well as injection with engineered antigen, or ingestion of antigen through the gut, are currently being tried. Engineered peanut protein allergies with modified IgE epitope binding sites have been used for immunizing mice with encouraging results.

Low rates of allergies to peanuts have been reported in countries where children normally eat peanut snacks certified as safe for infants. Desensitization involving giving patients small but increasing antigen doses in controlled environmental settings, followed by administration of regular maximum tolerated doses of antigen, has been induced in patients with food allergies by employing oral and sublingual immunotherapy techniques which are then followed by blinded or open food challenge with placebo or antigen.

Treatment protocols in experimental immunotherapies aimed at inducing tolerance in patients have been found to be safe, and any allergic reactions which have occurred have been managed by the use of epinephrine, steroids, and antihistamines. However, it is suggested that it is still unsafe to try such treatments involving immune tolerance induction in regular clinical practice because of the risk of severe reactions which can occur in patients.

Some immunotherapy strategies have involved the use of anti-IgE preparations. For example, patients with peanut allergies have been sort of treated in controlled studies involving injections with TNX-901 and Omolizumab (Xolair) which are both anti-IgE preparations. Also, a concoction of traditional Chinese herbs has been tried as a non-specific immunotherapy agent with promising results, especially in mice models. In other studies, the use of immunostimulatory sequences, for example, CpG motifs, have been found to reverse IgE-mediated sensitization from ragweed allergy in patients. Immunotherapy of reactions to insect stings and inhalants have also been attempted in animal models and human trials with some progress though understanding of the underlying immunological mechanisms is still needed.

There have been contradictory results about immunotherapy efficacy in many trials. This could be explained by differences in genetic make-ups of people (or animals in case of experimental studies involving animal models), types of allergy (e.g. seasonal), and the nature of allergies, e.g. types of foods eaten or the inhalants, or another environmental allergen. Management of patients with food allergies can only be possible if there is an adequate understanding of hypersensitivity versus tolerance in patients. Understanding of tolerance induction mechanisms and their effectiveness to various allergens, and knowing the best ways to deliver antigens, and whether long-term tolerance versus desensitization will ensue is still needed. Promises for novel immunotherapies for food allergies are currently being tried even before molecular mechanisms of immune tolerance are completely understood.

Engineered proteins that lack IgE-binding sites, engineered chimeric molecules bound to Fcg and allergen, co-administration with adjutants, e.g. heat-killed bacteria and CpG), which promotes T helper 1, and use of small overlapping peptides are some of the immune tolerance promoting immunotherapeutic methods which can avoid IgE binding-activation. Another potential method for delaying or preventing allergic syndromes is dietary manipulation and has been the subject of considerations and review. Exclusive breastfeeding of infants who are at high risk of developing allergic diseases for the first 3-6 months of life and nonuse of soy formula or cow milk supplementation have been found in some studies to prevent the development of allergic syndromes. American Academy of Pediatrics currently recommends that mothers of high-risk infants avoid foods e.g. nuts and seafood when they are still lactating.

Conclusions and Future Prospects

Food allergy is still a serious problem in the world today and maybe with us for some time to come unless concerted efforts in research are made. The problem is that we as humans have to eat and our immune systems must also react to the proteins and other components which get into our bodies. Therefore, the solution is to carry out more studies to understand the nature of immune tolerance and also how allergy to different molecular structures is developed. This requires not only a thorough understanding of immunological mechanisms but also molecular biology and biotechnology.

There is also the clinical aspect of allergic syndromes which must be managed by our clinicians, which also requires efforts by our biomedical scientists to develop modern and accurate diagnostic techniques. Not to be forgotten is the fact that some humans not only develop allergic reactions to food items but also to environmental, domestic, and occupational biotic and abiotic materials including pollen grains, mites, and even to rubber products, etc,which not only requires more research and understanding of the structures of the allergens but also of the underlying immunological mechanisms to develop modern disease management systems including immunotherapies.

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