Component-resolved Diagnostics in Allergy Practice Focusing on Food Allergy: A Systematic Review

Cow’s Milk

Cow’s milk (CM) is a significant cause of food allergy and anaphylaxis in children. It is a vital source of protein, energy, and calcium for the pediatric population. The prevalence of cow’s milk protein allergy (CMPA) during the first year of life ranges from 1.8 to 7.5%. This highlights the importance of recognizing and managing CMPA in pediatric populations. Also, CMPA can have significant implications for a child’s nutrition and overall health, making early detection and appropriate dietary interventions crucial for affected individuals.¹ CMPA is commonly diagnosed based on the clinical history of the patient, which raises suspicion along with different diagnostic tests such as the in vitro sIgE test and/or in vivo SPT. Cow’s milk contains various proteins with unique characteristics, including caseins, β-lactoglobulin, and α-lactalbumin, which are frequently identified as allergenic components. The majority of those with CMPA are allergic to these common CM allergens. Furthermore, the majority of patients afflicted with CMPA show hypersensitivity to both the whey and casein components. This information underscores the importance of identifying the specific allergenic components in CM when diagnosing and managing CMPA.^8,9 Cow’s milk proteins belong to the category of type I food allergens. These proteins have the ability to persist in the gastrointestinal tract, and if ingested, they can lead to allergic sensitization and trigger systemic allergic responses. This means that individuals with a milk allergy may experience allergic reactions throughout their bodies when even small amounts of CM proteins are consumed. It highlights the significance of understanding the potential risks associated with CM protein ingestion for those who are allergic.¹⁰ In the general population, various sensitivity patterns to CM allergens have been observed. Sensitivities to casein, β-lactoglobulin, and α-lactalbumin are often associated with each other, indicating a shared IgE response. On the contrary, sensitivity to bovine serum albumin (BSA) is unrelated to other CM allergenic components and has the potential of cross-reacting with beef proteins. Molecular diagnostics can be used to monitor CMPA and the development of tolerance to cow milk. This information highlights the complex nature of CM sensitivities and the potential for tracking tolerance using advanced diagnostic techniques.

A prospective study discovered that children diagnosed with CMPA and low levels of sIgE (sIgE) to components, such as α-lactalbumin, β-lactoglobulin, κ-casein, and αs1-casein, had a higher likelihood of developing tolerance to CM. This finding suggests that measuring the blood levels of sIgE to these specific CM proteins may be potentially utilized to predict the likelihood of tolerance development in children with CMPA.⁹ Consistent IgE epitope-binding patterns were observed in individuals with persistent cow’s milk allergy (CMA). The development of tolerance to CM was associated with a decrease in IgE epitope binding and an increase in equivalent immunoglobulin G4 (IgG4) epitope binding. This suggests that changes in the binding patterns of IgE and IgG4 antibodies to specific epitopes play a role in the development of tolerance to CM. These findings provide insight into the immunological mechanisms underlying the transition from allergic reactions to tolerance in individuals with CM allergy.^11,12 The monitoring of casein-specific and β-lactoglobulin-sIgE concentrations, along with the assessment of IgE/immunoglobulin G (IgG) ratios, can aid in predicting which patients are likely to develop tolerance to CM. By observing the levels of these sIgE antibodies and the balance between IgE and IgG antibodies, healthcare professionals can make informed predictions about the potential acquisition of tolerance to CM in patients. This monitoring approach provides valuable insights into the progression and potential outcomes of CM allergy, enabling personalized management strategies for individuals with CM allergies.¹³

Intense heating, such as baking, modifies the allergenic properties of CM proteins. Caseins, compared to whey proteins, are more resistant to heat. The allergenicity of β-lactoglobulin is reduced through the formation of intermolecular disulfide linkages and binding to other dietary proteins during the heating process.^14,15 Children who have a weak CMPA (IgE-mediated) usually tolerate extensively heated cow’s milk products. Conversely, children who react to the consumption of baked milk have a higher risk of experiencing a systemic reaction such as anaphylaxis and a more prolonged CMPA. In cases of chronic CMPA, children develop IgE antibodies against sequential cow’s milk protein epitopes, particularly casein. On the contrary, children who can tolerate and consume baked milk products produce IgE antibodies against the conformational cow’s milk protein epitopes, which are denatured via high temperatures.¹⁶ Elevated levels of sIgE antibodies targeting casein may suggest an absence of tolerance to baked milk. However, incorporating baked milk products into the diet of children with CMPA seems to accelerate the development of tolerance to CM.^17–20

The use of sIgE antibody levels can help identify patients who are at a higher risk of experiencing severe reactions during milk oral immunotherapy (OIT). Literature suggests that a thorough evaluation of IgE and IgG4 binding to CM peptides can predict the likelihood of a reaction to milk OIT and improve the safety of milk OIT by enhancing risk assessment.^21,22

Generally, CMPA has a positive prognosis, with most children becoming tolerant by school age. However, CMPA may persist into adolescence or adulthood if cow milk-sIgE (CM-sIgE) levels exceed 50 kUA/L. While CRD does not surpass traditional diagnostic methods using whole allergen extracts for diagnosing CMPA, it can be valuable in ascertaining tolerance development to milk that has been extensively heated and predicting the progression of CMPA, as well as the response on performing milk OIT^23–25 (Table 1).

Hen Egg

Allergic reactions to hen eggs are a common IgE-mediated reaction in children, affecting around 1–2% of children and exhibiting phenotypic variability. Egg allergy can have different phenotypes, including individuals who can tolerate eggs in highly cooked/baked items. Hen eggs contain various proteins, with Gal d 1–5 being the most commonly implicated allergens (Fig. 3).

Ovomucoid, in spite of being present in lower concentrations in egg whites compared to ovalbumin, is considered the dominant allergen in hen egg allergy. Testing for IgE antibodies against egg white proteins is commonly used for the diagnosis and management of egg allergy in children, as it includes the major allergens, such as ovomucoid and ovalbumin, making it a precise test for the initial diagnostic stage.^26–28 Multiple studies have suggested using cutoff values to diagnose egg allergy without conducting an OFC. However, no single cutoff value can provide a definitive diagnosis of egg allergy. Further research is necessary to establish diagnostic thresholds for heated and baked egg allergy.^29–31 Molecular diagnostics offers potential benefits in achieving a more accurate diagnosis of egg allergy, especially in assessing different clinical scenarios:

• Patients with clinical tolerance to hen eggs despite sensitization, typically show positive blood IgE tests to egg whites in the low to midrange, along with negative or low serum IgE tests to ovomucoid.

• Patients who tolerate and consume cooked eggs or processed foods containing cooked eggs often exhibit high serum sIgE levels to ovalbumin, similar to the levels observed in the test for egg white.

• Individuals allergic to all forms of egg, including raw and cooked, commonly have medium to high serum sIgE levels to egg white. Increased levels of serum sIgE to ovomucoid and ovalbumin may also be present.

Moreover, numerous toddlers have demonstrated tolerance to highly heated eggs, as extensive heating reduces the allergenicity of eggs. A significant percentage (64–84%) of children allergic to eggs can consume baked egg products without adverse reactions.³² Children with hen egg allergy frequently exhibit tolerance to the baked egg when it is consumed in a wheat matrix.³³ The introduction of a diet containing baked eggs has been observed to expedite the establishment of egg tolerance in contrast to complete avoidance. However, specific indicators for predicting the natural development of tolerance to cooked and raw eggs in egg-allergic patients have not yet been identified.

IgE reactivity to ovomucoid Gal d 1 is a potential predictor of clinical egg allergy.³⁴ Gal d 1-positive children are more prone to egg allergy, while those negative to Gal d 1 tend to tolerate eggs that have been boiled better. The sIgE levels to ovomucoid may aid in predicting the outcomes of food challenges involving eggs that have been cooked, but it does not replace egg white sIgE testing in evaluating egg allergy.³⁵ Sequential testing of IgE to egg white followed by IgE to ovomucoid significantly enhances diagnostic sensitivity compared to testing egg white alone.³⁶ Patients with IgE binding to conformational epitopes in hen eggs are more likely to resolve their allergy than those with IgE binding to sequential epitopes.³⁷ Additionally, ovalbumin-sIgG4 is an independent predictor of tolerance development to raw eggs. The ovalbumin-sIgE/sIgG4 ratio, along with SPT, can help identify individuals likely to tolerate both cooked and raw eggs.³⁸

Studies using murine models and observational human investigations have shown that a baked egg diet can lead to clinical remission of egg allergy and favorable immunological changes.³⁹ However, some studies do not support the immune-modifying effects of a diet involving baked eggs.³⁹ Due to the potential for systemic symptoms and anaphylaxis, the introduction of baked milk and eggs should be done under medical supervision.⁴⁰ Furthermore, OFC is often necessary for diagnosis and monitoring, but efforts are being made to explore alternative methods such as CRD, microarray-based platforms, and epitope mapping to potentially reduce the need for OFCs.^2,5 Serum IgE measurement or SPT of egg whites is currently recommended as the initial diagnostic test, with the use of molecular components being a promising approach to ascertain tolerance to eggs that have been cooked.² However, the final interpretation must be done by an allergist taking the patient’s clinical history into consideration² (Table 2).

Soybean Allergy

Soybean allergies in infants, particularly those with CMPA who consume soy-based formula, are attributed to gastrointestinal sensitivity.^3,41–43 Soybean contains multiple allergens, including Gly m 5, Gly m 6, and Gly m 8 from the seed storage proteins (SSP) family, which are stable to heat and digestion and associated with systemic responses.^41–43 Oral allergy syndrome (OAS) is linked to Gly m 4, a pathogenesis-related class 10 protein (PR-10), and can trigger allergic responses in individuals who are allergic to birch pollen.^5,44 Consumption of lightly processed soy by birch pollen–allergic individuals with Gly m 4 sensitivity may lead to severe reactions.⁴⁵ Soy is also found as a hidden allergen in various processed foods³ (Table 3).

Allergy to Peanuts, Tree Nuts, and Seeds

The IgE-mediated responses to nut proteins are common in peanut and tree nut allergies. There are two distinct types of nut allergies: (A) type I reactions, which encompass primary nut allergies characterized by severe and systemic reactions to nuts, and (B) pollen food syndrome (PFS) or OAS, which involves seasonal allergic rhinitis and mild symptoms in the mouth and throat when consuming fresh fruits, vegetables, or nuts.^46,47 Primary nut allergies are most commonly observed in children under the age of five.⁴⁸ The prevalence of nut allergy differs depending on the population studied, research methods and diagnostic criteria employed.⁴⁷ Peanut allergies have been reported to range in frequency between 0.5 and 2.5%, while tree nut allergies range between 0.2 and 2.2%.⁴⁹ Severe and prolonged symptoms frequently occur as a result of nut allergies. Severe non-fatal and fatal allergic responses, particularly among teenagers and young adults, are predominantly associated with nut allergies.⁵⁰ Peanut and tree nut allergies are the primary causes of anaphylaxis-related deaths in teenagers and young adults.⁵¹ Importantly, patients with a clinical history of asthma are at a heightened risk of encountering severe allergic reactions to food.⁵²

The clinical history of a patient is an important tool for identifying nut allergies. The occurrence of an immediate reaction after consuming peanuts or tree nuts, combined with positive sIgE testing, often provides enough evidence to confirm a suspected IgE-mediated allergy.⁴⁷ Diagnostic methods typically include SPTs or serum-sIgE tests. The size of the SPT wheal or sIgE titer level correlates with the likelihood of clinical allergy but may not be necessarily severe.⁴⁷ Additionally, CRD improves the accuracy of diagnosis and helps the assessment of both the risk and nature of the allergic response^1,2 (Table 4). The main allergen in peanuts is Ara h 2 and measuring Ara h 2 sIgE levels may provide better discrimination between the allergic children vs tolerant children compared to total peanut extract sIgE.⁵³ Threshold values for Ara h 2 have been established in various studies, with the predictive value varying across distinct populations. Measurements of Ara h 1, 3, and 6 are not as informative in this context. In situations where peanut sIgE is positive and sIgE Ara h 2 is negative, incorporating additional peanut components, along with the clinical context, can be helpful. On the contrary, isolated sensitivity to Ara h 8 (PR-10 protein and birch pollen allergen Bet v 1-homologue) is associated with milder or limited symptoms.⁵⁴ In certain regions such as southern Europe, the presence of lipid transfer protein (LTP) (Ara h 9) is linked to more severe systemic reactions and can serve as an indicator of severity.⁵⁵ Individuals with profilin or cross-reactive carbohydrate determinants (CCD) sensitization to peanuts typically experience minimal or mild oral symptoms, and they may tolerate heated peanuts.^2,5

Modern-day in vitro diagnostics is plagued with CCD-based false positive sensitizations that rarely result in clinical manifestations. These CCD components are present across the entire allergenic spectrum in pollens, venom, fruits, seeds, nuts, legumes, crustaceans, etc. Approximately 20% of the allergic population produce IgE antibodies to the CCD components which result in multiple positive results in an allergy report as a result of cross-reactivity.⁵⁶ The ALEX² (Macroarray Diagnostics) via the usage of molecular components and an inbuilt CCD inhibitor drastically minimizes CCD-based sIgE binding and false positive results, thereby preventing unwanted dietary restrictions and other management necessities.

Sensitization to the hazelnut components Cor a 9 and Cor a 14 exhibits higher specificity for primary hazelnut allergies when compared to hazelnut extract sIgE. However, the predictive values may vary among different populations.⁵⁶ Sensitivity to Cor a 9 and Cor a 14 significantly influences the threshold levels for hazelnut allergies⁵⁷ and is a predictor of more severe allergic reactions.¹³ The presence of isolated sIgE to Cor a 1 (PR-10, Bet v 1 homolog) is frequently associated with clinical tolerance or mild subjective oral symptoms. This indicates the likelihood of PFS rather than a primary nut allergy.⁹ When there is sensitization to both PR-10 nut components and seed storage components (such as Ara h 1, Ara h 2, Ara h 3, and Ara h 6 or Cor a 9 and Cor a 14), a thorough evaluation of the patient’s medical history is necessary. This combination suggests an underlying nut allergy,⁵⁸ emphasizing the need for a comprehensive assessment.⁵⁸ Reactions to nuts may indicate sensitization to non-specific LTP, such as Ara h 9 or Cor a 8, which have been associated with moderate-to-severe levels of systemic responses.⁵⁹ Sensitization to SSP such as Jug r 1, Jug r 2, or LTP (Jug r 3) is associated with severe reactions in individuals allergic to walnuts.⁶⁰ Ana o 3 has demonstrated strong predictive capabilities for cashew nut allergy, with sIgE to cashew components exhibiting superior diagnostic performance compared to cashew-sIgE or SPT specifically in children.⁶¹ Due to the wide repertoire of allergens involved in peanut allergies and other nut allergies, multiplex systems that incorporate various allergenic components such as the ALEX² (Macroarray Diagnostics) are crucial in providing a holistic image of an individual’s allergic status along with additional information such as risk stratification and prognostic information. The ALEX² has demonstrated significant sensitivity of 94.7%, and specificity of 95.7% in peanut-allergic individuals, while demonstrating a sensitivity of 100% in hazelnut patients and a specificity of 91.3%.

Limited research on sesame allergy in children is available in the current literature.⁶² Existing studies on sesame-allergic individuals have identified seven allergenic proteins, including five SSPs (Ses i 1, Ses i 2, Ses i 3, Ses i 6, and Ses i 7) and two oleosins (Ses i 4 and Ses i 5).⁷ Sensitization to rSes i 1 (a SSP) showed similar sensitivity to sesame-sIgE (86.1% for rSes i 1 vs 83.3% for sesame), but higher specificity (85.7% vs 48.2%) in a study involving 92 sesame-sensitized children.⁶³

The CRD testing serves multiple purposes, as it not only aids in assessing the probability of a positive OFC, but also offers valuable insights for distinguishing between primary anaphylaxis and pollen-related food allergies. Additionally, it assists in identifying cross-reactivity and co-sensitization among allergens.⁶⁴ Nonetheless, a recent systematic review that evaluated the diagnostic accuracy and risk assessment of CRD for food allergies revealed a limited number of studies available for each component, along with discrepancies in the cutoff values employed. This underscores the necessity for additional research in this area to enhance our understanding.⁶⁵ It is important to remember that clinical response may not always align with allergen sensitization, and therefore, all sIgE tests, including CRD, should be interpreted alongside the medical background of the individual.^56,58 Additionally, a recent study uncovered that the reliance solely on suggestive symptoms and positive IgE testing for the diagnosis of food allergies was only partially validated in comparison to the gold standard of a food challenge.⁶⁶

Food challenges, recognized as the gold standard, play a crucial role in definitively confirming or excluding a diagnosis when the patient’s medical history and sIgE test results do not yield a conclusive outcome.^47,67 When serological cross-reactivity, co-sensitization, and clinically relevant cross-reactivity or co-allergy cannot be adequately distinguished through sIgE tests, performing a targeted OFC specifically for nuts may be essential to establish a definitive diagnosis.⁶⁷ Therefore, to ensure optimal nutritional and medical care, the implementation of OFC should be tailored to the individual’s unique clinical circumstances.⁶⁸

Wheat Allergy

The IgE-mediated wheat hypersensitivity can present through diverse routes, encompassing oral intake (food allergy), inhalation (occupational asthma/rhinitis, e.g., baker’s asthma), cutaneous contact (contact urticaria), or subsequent physical exertion after wheat consumption [wheat-dependent exercise-induced anaphylaxis (WDEIA)].⁶⁹ Wheat sensitivity affects approximately 4% of preschool children.⁷⁰ The prevalence of wheat allergy rises from 2% to 9% among children aged 2–10 years, primarily attributable to secondary sensitization in individuals with a concurrent grass pollen allergy.^71,72 Nevertheless, primary wheat allergy generally emerges during early childhood and has a tendency to resolve by the age of 3–5 years.⁷³ Additionally, the estimated prevalence of wheat allergy among children under the age of three is up to 8%, whereas its occurrence is lower in adolescents and adults, impacting approximately 2% of the population.^74–77 Among bakery employees, Baker’s asthma is observed in approximately 1–10% of individuals, with a higher occurrence in men.^78–81 Furthermore, WDEIA often manifests in teenagers and young adults who consume products made from wheat and engage in physical activity.⁸² To date, researchers have identified 28 allergenic components in wheat grains^2,7,83,84 (Table 5). The albumin/globulin (A/G) fraction is composed of α-amylase/trypsin inhibitors (Tri aA_TI), the non-specific LTP Tri a 14, and the wheat serpin Tri a 33. Within the gluten fraction, there are components such as Tri a 19 (omega-5 gliadin), along with the high molecular weight glutenins Tri a 26 and the low molecular weight glutenins Tri a 36.^2,84 Also, Tri aA_TIs play a role in both food allergy and WDEIA. Furthermore, Tri a 14 is a food allergen associated with baker’s asthma in Italian individuals with wheat allergy, while Tri a 33 is implicated in both respiratory and food allergies to wheat.^2,84 Wheat gliadins are regarded as markers of true wheat sensitization, with Tri a 19 being the primary allergen in WDEIA and an important allergen in young infants experiencing acute adverse immune responses to consumed wheat.⁸⁵ Furthermore, Tri a 36, which is a major allergen in individuals with WDEIA, exhibits an elevated expression during the development of wheat seeds. It contains a heat- and enzymatic digestion-resistant domain.⁸⁶ When evaluating patients suspected of wheat allergies, a comprehensive medical history is always obtained, which includes assessing for respiratory allergies triggered by pollen. In addition, diagnostic measures such as SPT with wheat, detection of sIgE for the relevant allergens, and analysis of various molecular components such as wheat, gliadin, rTri a 14, and Tri a 19 are conducted.²

Fruits and Vegetable Allergy

Adolescents and adults commonly experience allergies to fruits and vegetables. The identification of allergens associated with cross-reactivity patterns has contributed to an improved understanding of sensitization processes and the impact of allergen profiles on different phenotypes.⁸⁷ Fruits and vegetables can cause allergies from initial sensitization to food allergens via the gastrointestinal tract or subsequent sensitization to cross-reactive food allergens due to pre-existing sensitization to related latex and pollen allergens.⁸⁸ The predominant clinical presentations of fruit and vegetable allergies are characterized by LTP and PFS syndrome.⁸⁷ Furthermore, PFS, also known as OAS, is an allergic response to plant-based foods that results in itching of the lips, tongue, and mouth. Unlike typical food allergies, OAS necessitates prior sensitization to a cross-reactive inhalant allergen before becoming sensitized to a specific protein in the food.⁸⁹ Proteins found in pollen and certain plant-derived foods can trigger an IgE-mediated response in OAS due to their structural similarity. In individuals with grass allergies, consuming raw foods such as melon, orange, or tomato can result in an allergic reaction. However, not all pollen-sensitive patients experience cross-reactions and PFS-like symptoms.⁹⁰ In PFS, allergies from vegetables and fruit primarily cause sensitization that occurs through exposure to unstable pollen allergens such as PR-10 (Bet v 1 type allergen) or profilins, leading to mild local oropharyngeal responses. Conversely, LTP syndrome is associated with sensitization to persistent plant–food allergens called LTPs, which can lead to recurrent systemic reactions and potentially even anaphylaxis.⁸⁷ Fruits from the Rosaceae family, including apricot, apple, peach, strawberry, pear, and raspberry, are recognized for triggering adverse reactions.⁹¹ These fruits can be ingested in both fresh or processed, and allergenic compounds can be found in both the peel and pulp.² Information regarding the prevalence of allergies to fresh fruits is limited. with estimates ranging from 0.1 to 4.3% in a comprehensive evaluation of overall fruit allergy incidence.⁹² Among the general population, peaches exhibit the highest allergic sensitization (7.9%). Subsequently, apples account for 6.5% of reported allergies, followed by kiwis at 5.2%.⁹³ Similarly, vegetables, and fruit can also exacerbate allergy symptoms in hypersensitive individuals.⁹⁴ Foods belonging to the Apiaceae family, including celery and carrot, are widely recognized as potential allergens and are commonly consumed in both raw and cooked forms.⁹⁵ Table 6 demonstrates that the majority of plant–food allergens can be classified into three protein families, namely, PR-10, LTP, and profilins.⁹⁶ Within the PR-10 family, the primary allergen is found in fruits of the Rosaceae family (e.g., Pru p 1 in peach and Mal d 1 in apple).⁹⁷ Similarly, the PR-10 protein in carrots and celery, which belongs to the Apiaceae family, is a significant allergen, particularly in Central Europe.⁹⁸ These allergenic proteins are present in both the pulp and peel of fruits and vegetables. They are sensitive to heat and low pH, and their production is influenced by environmental stress and pathogen attacks.⁹⁹ Generally, these allergens tend to cause mild responses in the oral cavity, although the allergenicity can be affected by fruit processing methods such as the pasteurization of juices and jams.⁹⁶ On the contrary, LTPs are small proteins characterized by a compact tertiary structure comprising four disulfide bridges. These proteins play a crucial role in the transportation of lipids across cell walls.¹⁰⁰ These allergens can be found in different fruits, including apricots, apples, peaches, plums, cherries, raspberries, pears, blackberries, strawberries, and others, predominantly in the surface tissues such as the peel.¹⁰¹ They are proteins that are heat resistant and acidic pH and are known to be upregulated in response to pathogen attacks.¹⁰⁰ These unique characteristics of the allergens can lead to generalized systemic reactions in individuals who are sensitized.¹⁰² Celery contains two types of non-specific lipid transfer proteins (nsLTPs). Also, nsLTP type I (Api g 2) is expressed in the stalks, while nsLTP type II (Api g 6) is expressed in the tuber.^103,104 In populations living in the Mediterranean region, there is a higher occurrence of sensitization rates to LTPs compared to those residing in northern Europe, where sensitization to PR-10 proteins is more prevalent.^105,106 The variation in sensitization rates between these regions is associated with higher exposure to Fagales tree pollen (such as alder, birch, and hazel) and subsequent sensitization to Bet v 1.² Profilins, which are small and widely distributed proteins found in plants, are involved in cellular signal transmission pathways and exhibit low to intermediate heat stability.¹⁰⁷ Profilin sensitization is prevalent, but its clinical significance is limited to a few cases.¹⁰⁸ Allergies to apples, peaches, pears, and strawberries have been associated with this type of sensitization.¹⁰⁹ In the case of vegetable allergies, profilin is considered a significant sensitizer for individuals allergic to celery, while its impact is minor in carrot allergies.² The prevalence of profilin sensitization is evenly distributed but more common in the Mediterranean region.² Biotic and abiotic stressors contribute to the production of thaumatin-like proteins (TLPs), which possess a rigid three-dimensional structure formed by eight disulfide bridges from cysteine residues. Allergies to peaches, kiwis, apples, and cherries are attributed to these proteins.¹¹⁰ Based on evidence from peaches, apples, and cherries, TLPs are classified as mild allergens.² Panallergens such as Bet v 1-related food proteins, profilins, and nsLTPs exhibit significant cross-reactivity across various plant species.¹¹¹ These protein families have been linked to various clinical manifestations, such as the development of OAS and anaphylaxis. Notably, nsLTP-mediated fruit allergies tend to elicit systemic responses more frequently than those mediated by Bet v 1 or profilin.² The presentation of fruit allergies can differ based on the attributes of allergen families and the mode of sensitization. Patients sensitized to plants in the Betulaceae family may develop sIgE antibodies against Bet v 1-homologous proteins found in various fruits from the Rosaceae family.⁹⁹ The ingestion of raw fruits commonly results in moderate symptoms predominantly confined to the oral cavity (OAS).⁹⁹ In some cases, individuals initially sensitized to peaches (with Pru p 3 acting as a sensitizer) may acquire cross-sensitization to other fruits containing LTPs, leading to sensitization to non-specific LTPs.^101,102,112 The ALEX² Allergy Explorer (Macroarray Diagnostics) based on a multiplex microarray system has demonstrated significant sensitivity (94.1%) and specificity (100%) in the diagnosis of peach allergies due to its ability to test for multiple peach allergenic components simultaneously.¹¹²

Clinical presentations can vary from localized symptoms to severe anaphylaxis, also known as “LTP syndrome.” The clinical presentation can be influenced by co-factors such as alcohol consumption and medication use, or physical factors such as exertion.¹¹³ LTPs are one of the primary causes of anaphylaxis caused by food in Italian adults, although the percentage of sensitized patients who developed sensitivity or hypersensitivity experiencing anaphylactic episodes is less compared to allergens such as almonds, peanuts, or prawns. This has led some researchers to consider LTPs as a potentially hazardous yet “harmless” allergen.¹¹¹ Patients who have developed sensitization to profilins present in grass pollen (Poaceae family) may experience cross-reactivity to the profilin found in fruits from the Rosaceae family.¹¹⁴ However, profilin sensitization often remains clinically silent.¹⁰⁸ When symptomatic, the main clinical manifestation is OAS, with a low to moderate risk of a systemic allergic response.¹⁰⁹ Kiwifruit allergies can be initiated either through primary sensitization due to gastrointestinal food allergy or through cross-reactivity with allergens from birch or grass pollens and latex. This cross-reactivity occurs because of a similar protein found in kiwifruit, which bears a resemblance to Hev b 11, a latex chitinase.² Kiwifruit allergy symptoms vary in severity, from mild symptoms in the mouth and throat to serious systemic reactions. Actinidin (Act d 1) is the main allergen found in kiwifruit and is closely linked with sensitivity to this fruit.¹¹⁵ Research has shown that allergen sensitization to Act d 8 and Act d 9 is specific to individuals suffering from pollen-kiwifruit allergies.¹¹⁵ The cross-reactivity between kiwifruit nsLTP (Act d 10) and other nsLTPs is minimal due to a lack of significant homology.¹¹⁶ Other fruits such as avocado, mango, chestnut, and banana have shown cross-reactivity with latex allergens, resulting in a condition termed “latex–fruit syndrome” (LFS).² First identified in 1994, LFS is a hypersensitivity reaction to certain raw fruits. This condition impacts around 30–50% of individuals with a natural rubber latex (NRL) allergy and involves IgE antibodies that identify similar epitopes on closely related proteins.^117,118 A total of 15 latex allergens, known as Hev b 1 to Hev b 15, have been discovered, with LFS connected to four of these allergens (Hev b 2, Hev b 6.02, Hev b 7, Hev b 8, and Hev b 11).^7,119

Seafood and Shellfish Allergy

Various allergens, which are stable, water–soluble proteins, have been found in seafood, including fish and shellfish, with most of them residing in the consumable part of these creatures.¹²⁰

Meat Allergy

Allergies to mammalian meat are infrequently examined as they primarily affect young atopic infants, triggering rapid allergic responses following exposure.^125,126 Recently, novel meat allergy entities causing delayed reactions have been recognized, primarily in adult populations.^125,127 Bovine species possess a significant number of food allergens, with nine allergenic proteins identified thus far.¹²⁵ However, as most reported beef reactions have been observed in children allergic to CM,¹²⁸ many of these allergens were initially identified as CM allergens.¹²⁵ Research indicates that approximately 10% of CM-allergic children manifest clinical reactions upon consumption of beef.¹²⁹ Although BSA (Bos d 6) and IgG (Bos d 7) are significant beef allergens, BSA seems to be more relevant in these reactions.^125,130 Thus, in the diagnostic process for CM-allergic children, investigation of sIgE to Bos d 6 may assist in identifying those at risk for beef-induced reactions. Another meat allergy phenomenon is termed “cat–pork syndrome,”¹³¹ where sensitization to domestic furred animals (commonly cats) can lead to IgE-mediated hypersensitivity reactions upon pork consumption.¹²⁵ Cross-reactive serum albumins (66–69 kDa) from animals, such as Fel d 2 in cats and the porcine meat allergen Sus s 6,¹³² can account for this reaction. Fel d 2, a 67-kDa serum albumin, is a minor cat allergen, sensitizing only about 15–35% of cat-allergic individuals.¹³² Considering that approximately 30% of individuals sensitized to Sus s 6 experienced allergic symptoms after eating pork, only 1–3% of cat-allergic patients seem to be at risk for a pork meat allergy.¹³² Recently, a delayed allergic reaction post-consumption of mammalian meat has been documented. This form of IgE-mediated allergy is attributed to a novel relevant carbohydrate allergen, galactose-α-1,3-galactose (α-gal), triggered by tick bites.¹²⁹ The identification of α-gal was based on the observation of individuals experiencing acute anaphylaxis following their first exposure to the monoclonal antibody cetuximab.¹³³ Investigation into cetuximab-sIgE antibodies unveiled that these antibodies specifically targeted oligosaccharide residues on the heavy chain, with α-gal being the primary epitope.¹³⁴ Also, α-gal, a non-primate mammalian glycan bearing resemblance to B-group blood antigen, is present in all forms of mammalian tissues and products such as red meat, kidney, gelatin, milk, cheese, and gelatin-based vaccinations.^123,124 Specific IgE to α-gal has been associated with delayed angioedema, urticaria, and anaphylaxis post-red meat consumption.^135,136 Any products containing α-gal could potentially trigger reactions in susceptible individuals, as recently evidenced by a case of anaphylaxis following the administration of a gelatin-containing vaccine in a sensitized pediatric patient.¹³⁷ The diagnosis of this allergy can be challenging due to the common 3–6-hour delay between mammalian meat consumption and symptom onset. Most individuals develop this allergy after years of uneventful beef or pork consumption.^126,136 Recent studies suggest individuals with B-group blood antigens are less likely to develop α-gal sensitivity.¹³⁸ The immunological mechanism underlying sensitization and delayed symptoms remains largely unknown, with tick bites being the only recognized sensitization pathway.¹²⁵ The implicated tick species include Amblyomma americanum (USA), Ixodes holocyclus (Australia), and Ixodes ricinus (Europe).¹²⁵ Data supporting the role of tick bites in α-gal sensitization process is heterogeneous; there is epidemiologic evidence demonstrating increased sIgE to α-gal following confirmed tick bites, and geographical correlation exists between areas with high prevalence of tick bites and the global distribution of delayed anaphylactic reactions to red meat.¹³⁹ Specifically, Hamsten et al. illustrated that α-gal is present in the gastrointestinal tract of I. ricinus, potentially leading to exposure to α-gal during a tick bite.^140–142 In conclusion, the development of sIgE to α-gal represents a new cause of food allergy and anaphylaxis subsequent to meat consumption, predominantly occurring in adults but also reported in children. It is characterized by a delayed onset of symptoms, a diet devoid of red meat, and a history of tick bites.^126,136 Future research should consider the potential of IgE-mediated allergy to α-gal in instances of urticaria, angioedema, or anaphylaxis occurring 3–6 hours following red meat consumption (Table 8).