Diagnosing food allergy

What is oral tolerance?

Oral tolerance is the process of immune unresponsiveness to orally ingested antigens. It is initiated when antigenic material is sampled from the intestinal lumen by CX3CR1 macrophages, which is then processed by non-activated dendritic cells (Figure 1)^1,2.

These non-activated dendritic cells then present the food peptide, displayed on the major histocompatibility complex (MHC), to the T cell receptors (TCR) on naïve T cells^1,2.

This causes naïve T cell differentiation into T regulatory cells (Tregs), which then travel via α4β7 to the lamina propria. These Tregs support the maintenance of tolerance via CTLA-4 expression and the release of transforming growth factor-beta (TGF-β) and interleukin (IL)-10^1,2.

Pathogenic type 2 helper T (T_H2) cells play a central role in the pathophysiology of allergic disease³

CTLA04 then suppresses antigen-specific T_H2 cells, while TGF-β and IL-10 inhibit effector cells: eosinophils, basophils, and mast cells. In the presence of IL-10 and TGF-β, B cells produce IgA and IgG4 that are necessary for maintaining tolerance while suppressing IgE production^1,2.

An intact epithelial barrier also plays an essential role in the maintenance of tolerance and the loss of integrity of this barrier can lead to sensitisation to food allergens and the development of food allergy^1,2.

Figure 1. Overview of the tolerogenic (left) and allergic (right) response to food antigen in the gut (Adapted¹). CTLA-4, cytotoxic T lymphocyte-associated antigen-4; CX3CR1, C-X3-C Motif Chemokine Receptor 1; DC, dendritic cells; IgA, immunoglobulin A; IgE, immunoglobulin E; IL, interleukin; ILC2, type 2 innate lymphoid cells; MHC, major histocompatibility complex; TCR, T cell receptors; TGF-β, transforming growth factor-beta; T_H2, type 2 helper T cells; T_H9, type 2 helper T cells; Tregs, T regulatory cells; TSLP, thymic stromal lymphopoietin.

What is sensitisation?

Allergic sensitisation is the first step of the allergic immune response following exposure to an allergen. It is defined by the presence of detectable food-specific IgE (in blood or by skin prick test), which can lead to the development of clinical food allergy^1,2.

Sensitisation occurs when a food allergen triggers the secretion of pro-inflammatory cytokines, such as IL-33, IL-25, and thymic stromal lymphopoietin (TSLP). This promotes the expansion of type innate lymphoid cells (ILCs) and activates dendritic cells that usually defend against pathogens^1,2.

Activated dendritic cells then take up and process the antigen into short peptides in addition to upregulating the expression of the surface protein OX40L. The interaction of MHC and OX40L on dendritic cells with TCR and OX40 on naïve T cells then triggers the differentiation of naïve T cells to T_H2 cells, which are central to the pathophysiology of food allergy^1,2.

What are the effector cells of allergic reactions?

Following differentiation, T_H2 cells and ILCs secrete pro-inflammatory cytokines, such as IL-5 and IL-13, which promotes the recruitment of eosinophils and basophils. T_H2 cells also secrete IL-4, which prompts food allergen-specific B cells to class switch, leading to amplified production of food antigen-specific IgE antibodies and a state of sensitisation and allergy^1,2.

T_H9 cells are also involved in the development of the allergic response through the secretion of IL-9, which promotes the recruitment of mast cells^1,2

Antigen food-specific IgE binds to FcεRI receptors on mast cells and basophils. On subsequent exposure to the allergen, the allergen binds two or more IgE molecules leading to IgE cross-linking. This stimulates preformed mediators to release into the circulation, leading to the allergic cascade and manifestation of allergic symptoms. These include mediators important in the induction of anaphylaxis: histamine, tryptase, platelet activating factor, prostaglandins, and leukotrienes^1,2.

Why is the understanding of food allergy pathophysiology important?

As the understanding of food allergy continues to evolve, greater insight into IgE-mediated food allergy pathophysiology may offer new opportunities for the treatment and diagnosis of food allergy.

• The investigation of T_H cell biology has led to the organisation of T_H cells into distinct subsets - a notable example includes a new subset of follicular T_H cells being described³
• Detection of IgE+ cells in the stomach and duodenum has defined the gastrointestinal tract as a reservoir of IgE+ B-lineage cells in food allergy⁴
• Multivariable models indicate that IgE diversity to peanut allergens and peanut IgE specific activity have a large influence on effector cell activation⁵

What is the key information to collect from a clinical history?

A comprehensive clinical history is essential for establishing the pre-test probability of food allergy and guiding further investigations⁷. A detailed clinical history, using structured questions, is recommended by various international guidelines for the diagnosis of food allergy^9,10. As part of a clinical history, there are numerous important pieces of information to collect (Table 1), including dietary history, timing and chronicity, reproducibility, known risk (co)factors, and coexisting conditions^10,11.

Table 1. Key information to collect from a clinical history (Adapted¹¹). NSAID, non-steroid anti-inflammatory drug.

Food allergy can also lead to poor growth in children, necessitating the intervention of a dietician. A physical examination should therefore assess the growth and development of children, in addition to their nutritional status, especially in children with multiple food allergies⁷.

What conditions induce a similar clinical manifestation to IgE-mediated food allergy?

It is also important to consider alternative diseases, triggers, and syndromes with similar clinical manifestations to IgE-mediated food allergy, such as^11,12:

• Medications or insect stings
• Metabolic disorders, including gluten and lactose intolerances and sensitivities
• Food poisoning and toxic reactions, such as histamine in scombroid poisoning
• Chemical exposures, such as chlorine or fragrant perfumes
• Viral syndromes that may cause rhinorrhoea and/or urticaria
• Reactions and sensitivities to food additives
• Acute spontaneous urticaria and angio-oedema
• Food refusal or aversion (notably in young children)

How does exercise induce anaphylaxis?

Food dependent exercise induced anaphylaxis (FDEIA) is a disorder in which exercise induces allergic symptoms¹¹.

It is diagnosed when a patient has the signs and symptoms of recurrent anaphylaxis two to four hours following the consumption of a food allergen, during or after physical exertion¹¹.

Both exercise and food allergen ingestion should not individually produce any symptoms and sensitisation to the food allergen should be demonstrated using skin prick or in vitro (specific IgE) testing. A provocation test combining food ingestion with exercise may be required to confirm the diagnosis¹¹.

What is Oral Allergy Syndrome (OAS)?

Patients with oral allergy syndrome frequently have concurrent allergic rhinoconjunctivitis due to sensitisation to airborne pollens. It results from IgE sensitisation to aeroallergens and antibodies recognising epitopes in fruit, vegetables, and other plant-derived food proteins¹¹.

Cooking or processing of food can denature these proteins, resulting in the reduction of the IgE-binding avidity and a reduced immune response when consumed. If more severe symptoms arise, such as vomiting, diarrhoea, and anaphylaxis, this would be more consistent with systemic reactions associated with heat-stable proteins¹¹.

How are non-IgE or mixed mediated reactions diagnosed?

The nature of non-IgE mediated adverse food reactions means that the diagnostic techniques for detecting food-specific IgE are not useful to confirm the diagnosis. The diagnosis of these reactions normally relies on elimination diets and the oral food challenges¹¹.

Mixed IgE- and non-IgE-mediated mechanisms are associated with the manifestation of symptoms in the gastrointestinal (GI) tract and skin¹¹. An example of a mixed IgE- and non-IgE-mediated reaction is eosinophilic esophagitis (EoE). Currently EoE is diagnosed using upper endoscopy with 2–4 biopsies. More research is needed to determine if there is a clinical benefit to using serum IgE (sIgE) and skin prick test (SPT)¹¹.

What diagnostics are commonly used for diagnosing food allergy?

Two commonly utilised standardised tests to determine the causative food are the skin prick test (SPT) and the measurement of total and allergen-specific sIgE levels in whole extracts (Figure 2)⁷.

Figure 2. The diagnostic tests used to diagnose IgE-mediated food allergy utilise specific aspects of the underlying mechanisms of sensitisation (Adapted¹³). IgE, immunoglobulin E; IgG, immunoglobulin G.

The skin prick test entails pricking the skin with various candidate food allergens to measure the response of cutaneous mast cells to allergen. A wheal and flare response within 15 minutes suggests the presence of cutaneous mast cells with allergen specific bound IgE^13,14.

By contrast, the measurement of total IgE or sIgE utilises fluorescent enzyme immune assays to detect the presence of either total or serum IgE to allergen extracts or individual allergen components. Ratios with allergen sIgE can be calculated using total IgE and allergen-specific IgG4^13,14.

Benefits of BAT/MAT testing over conventional skin prick tests

Dr Sharon Chinthrajah describes the benefits of utilising basophil activation tests (BAT) and mast cell activation tests (MAT) over conventional skin prick tests in the diagnoses of food allergies.

The basophil activation test (BAT) is an emerging test that measures the response of circulating basophils to allergen that has shown high sensitivity and specificity, but is not yet fully commercialised¹⁵.

In isolation, SPT and sIgE should not be considered diagnostic as they indicate IgE sensitisation but not necessarily the development of a clinically significant food allergy^9–11

Careful patient selection, with diagnostic testing guided by clinical histories, is advised as these tests are associated with a high rate of false positive or false negative diagnoses that can lead to unwarranted dietary restrictions or delayed diagnostic workups^9–11.

Diagnoses should then be confirmed with oral food challenges or elimination diets assessing the clinical symptoms associated with the ingestion of the suspected food allergen¹¹.

What are the predictive values of SPTs and sIgE?

The threshold values for SPTs and sIgE have been determined for several allergens (Table 2) that define the 95% Positive Predictive Value (PPV) likelihood of patients having a reaction during a food challenge¹¹.

Table 2. >95% positive predictive value for SPT weal diameter and sIgE level for a positive oral food challenge. SPT, skin prick test; sIgE, specific IgE (Adapted⁷).

Compared with the oral food challenge, SPTs have a low specificity and Positive Predictive Value for an initial diagnosis of food allergy⁷. Moreover, SPTs have a high sensitivity and high negative predictive values (NPV>90%), which can be a valuable tool for excluding food allergy⁷.

However, it is possible to have negative SPTs in patients with IgE-mediated food allergy. In cases of negative SPTs where clinical histories suggest food allergy, further evaluation and OFC should be carried out⁷.

In a systematic review (N=2,831) investigating the accuracy of tests used to diagnose food allergy, both sIgE and SPT showed reasonable sensitivity (70–100%) and moderate specificity (40–70%) using the double-blind, placebo-controlled food challenge as a reference standard¹⁶. However, the sensitivity and specificity of both diagnostic tests varied due to differences between studies and the specific food being studied¹⁶.

While no definitive algorithm for determining the necessity of and OFC has been found, alternative methods for diagnosis have been developed¹⁷.

A notable example was the first study to evaluate the utility of combining optimised threshold values for SPT with sIgE or the specific IgE / total IgE ratio (sIgEr). In this study (N=427), the combination of SPT cut-off values with sIgEr resulted in greater area under curves (AUCs) than SPT, sIgE, or sIgEr alone in the prediction of food challenge outcome¹⁷.

What are elimination diets and the oral food challenge?

Elimination diets involve the avoidance of foods that are suspected of triggering the allergic reaction based on clinical history and diagnostic testing. Usually lasting 2–4 weeks for IgE-mediated symptoms, the results of the diet can be used to help inform the diagnosis^9,10.

When the elimination of food does not lead to an improvement in symptoms, food allergy is unlikely. The avoidance can then be followed by a reintroduction of eliminated foods.

Clinical reaction to a food should be confirmed with an oral food challenge (OFC) under medical supervision^9,10. OFCs are considered the gold standard for confirming the diagnosis of a food allergy, monitoring food allergies, or proving oral tolerance. They are preferably conducted using a double-blind, placebo-controlled approach with increasing doses of the suspected food allergen ingested over fixed intervals, until a reaction is observed or the maximum dose is reached¹¹. For clinical purposes, open OFCs are acceptable, especially in children. This diagnostic technique is however limited in its use outside of specialist centres due to its resource intensive nature and the potential risk of a severe anaphylactic reaction¹⁸.

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