Introduction

The last few decades have witnessed an increased prevalence of the allergic diseases worldwide, and food allergy constitutes a major part of this increase [1, 2]. Our gastrointestinal tract is continuously exposed to a large number of antigenic loads in the form of either commensal bacteria, harmful antigens, or food antigens. There is a need to balance between the immune reactions and tolerogenic reactions. Failure of this balance or immune tolerance to food antigens results into a strong immune reaction against the ingested foods and may cause allergy-like symptoms. Basically, food allergy may be defined as a disorder characterized by adverse immune response upon the ingestion of even a tiny amount of food and food components in the susceptible individuals. It is important to note that not all the constituents of food are responsible for the allergic reactions, but only a few proteins present in food are recognized by the immune system as foreign and are responsible for eliciting an immune response [3]. Food-induced allergic reactions are the consequence of the immunological responses against any food protein that may include effector cells, food-specific IgE antibodies, and cell-mediated reactions resulting into acute, sub-acute, and in some cases chronic reactions. Further, food-induced allergic reactions include symptoms involving the respiratory tract, gastrointestinal tract, and skin in the susceptible individuals [4, 5].

The anaphylactic reactions due to consumption of food affect a significant percentage of the population, and the prevalence of food allergy is estimated about 6–8% in children and 3–4% in adults [6]. The incidences of food allergy vary from country to country and depend on the food habits as well as lifestyle of a particular geographical area like soy allergy is common in Japan; peanut allergy in the UK, France, and North America; and sesame allergy in Israel [7]. The eight foods commonly known as the “Big Eight” account for about 90% of documented food allergy and includes peanuts, soybeans, cow’s milk, hen’s egg, fish, crustacean, wheat, and tree nuts. Out of these foods, cow’s milk, eggs, wheat, soy, peanut, and fish are the main causes of food allergy in infancy and early childhood [8,9,10]; whereas peanut, tree nuts, and seafood are reported to be causative factors in 85% of severe reactions in grown-up children and adults [11, 12]. In addition to these common and potential allergens, few uncommon or unusual foods may also cause allergic reactions in sensitive individuals; these include mustard [13], sesame seeds [14], mango [15], red meat [16], avocados [17], kiwis [18], spices and condiments [19], banana [20], cereals [21], eggplant [22], and many others.

In the recent years, the scientific community and clinicians have focused their attention towards mustard allergy as the incidences are increasing day by day. Basically, mustard belongs to the Brassicaceae family that includes more than 3200 species and 375 genera together with some important vegetable crops. The most common varieties of mustard are as follows:

  1. 1.

    White mustard (Brassica alba; syn.: B. hirta, Sinapis alba), also known as yellow mustard and is cultivated for its pungent seeds. It is a common source of table mustard

  2. 2.

    Black mustard (B. nigra; syn.: S. nigra, B. sinapioides), a Eurasian annual variety of mustard with pungent odor and flavor is an important source of oil. Its seeds are used in the pharmaceutical industry for cataplasms because of their revulsive properties

  3. 3.

    Brown mustard (B. juncea; syn.: B. integrifolia), commonly known as oriental mustard, Chinese mustard, Indian mustard, leaf mustard, Sarepta mustard, and Asiatic mustard that is used as a spice and a potherb and for mustard oil.

Mustard is widely used in foods to enhance the flavor and for its nutritional values; therefore, there are chances of provoking allergic manifestations in susceptible individuals (Table 1) [23]. Foods formulated with mustard are expected to increase in popularity in the future due to its sensory attributes, its high protein content, and its functional properties (Table 2) [24, 25]. The widespread use of mustard in foods has raised concerns that mustard can cause IgE-mediated allergic reactions in sensitive individuals. Allergy to mustard depends upon the extent of its consumption, and prevalence is rather high in the areas where the consumption of mustard is more than in other parts of the world.

Table 1 List of foods containing mustard
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Table 2 Mustard nutrient compositions
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Scenario of Mustard-Induced Food Allergy

Mustard allergy accounts for 1.1% of food allergies in children [26] and about 6–7% of total food allergies [27]. Mustard-induced anaphylaxis has also been reported over the last 20 years [28, 29].

In countries like France, mustard is extensively consumed in foods as sauce or food additive with the consequent risk of mustard allergy. Mustard is the fourth leading cause of food allergy in France after milk, eggs, and peanuts. In France, about 10% of school children are allergic to at least one or more food items and the most common food allergens include hen’s egg (35%), peanut (24%), cow’s milk (8%), mustard (6%), and fish (4%) [30,31,32]. Rance et al. investigated 36 children with positive mustard SPT. Children (51.8%) had atopic dermatitis and 37% experienced urticaria and/or angioedema. Children were allergic to one or more food items including peanut, egg, soy, hazelnut, beef, shrimp, wheat, avocado, and cow’s milk [29]. Similarly, one more study from France included 378 children with food hypersensitivity and it was observed that five allergens accounted for 82% of confirmed food hypersensitivity: egg (51.8%), peanut (34.3%), milk (11.6%), mustard (8.9%), and codfish (7.1%) [33]. Another study conducted by Rance et al. included 42 children with cashew allergy. Out of these 42 children, 3 were sensitized to mustard [34]. A study on 163 asthmatic children with food allergy indicated that 6.9% were positive to mustard with mixed symptoms affecting the gastrointestinal tract, respiratory tract, and skin [35]. Mathelier-Fusadeet et al. reported seven cases of food-dependent exercise-induced anaphylaxis, and mustard was mentioned as one of the responsible foods [36]. Further, Andre et al. (1994) screened 580 patients with pathological reactions to foods and found that mustard (3%) is one of the foods that were responsible for anaphylactic reactions and it is often present as hidden allergen in commercial foods [37]. Similarly, Morisset et al. conducted a study in France for screening of mustard allergy in 30 subjects (3–20 years) by SPT using ground mustard seeds (B. nigra), mustard flour (B. juncea), metabisulfite-free strong mustard seasoning (B. juncea), and a commercialized allergenic extract (B. nigra) and it was observed that about 23.3% of the sensitized subjects were allergic to a routine dose of mustard [13]. Furthermore, a case study in France revealed that a 38-year-old woman with history of mustard allergy showed symptoms of anaphylaxis 20 min after ingestion of chicken dips containing mustard as an ingredient. SPT results showed positive reactions to mustard, coriander, and curry powder (which contained mustard). The woman had 56.3 kU/L mustard-specific serum IgE levels [38]. Additionally, in two individuals, life-threatening exercise-induced anaphylactic reactions to mustard have also been reported in France. One patient was allergic to wheat, parsley, tomato, carrot, peanut, and hazelnut while the other patient was allergic to soybean, peanut, hazelnut, and strawberry [39]. Further, a 51-year-old woman had two hospitalizations within 8 months for acute generalized urticaria, with lip and tongue edema and dyspnea, which appeared within 20 min of eating fillet of anchovy in sauce. SPT was done for mustard and fish that was positive to the former [40].

In Spain, where mustard is widely used, only a few reports are available in reference with mustard allergy and no large-scale studies have been carried out or published so far. A study conducted with 29 patients with history of mustard allergy underwent skin prick tests with mustard and were found positive. All patients had high levels of mustard-specific IgE (0.7 to > 100 kU/L). Out of 29 patients, 19 (66%) showed a systemic reaction after consumption of mustard, 10 (34%) had a local reaction, and 14 (48%) had anaphylaxis [41]. Similarly, a study was performed by Figueroa et al. in which 38 patients were screened for mustard allergy by SPT and 24 patients by double-blind placebo-controlled food challenges (DBPCFC). Out of the 24 subjects, 14 cases showed positive results to DBPCFC and showed significantly greater mustard SPT than those of controls [42]. Moreover, a 47-year-old woman with history of seasonal rhinoconjunctivitis showed symptoms of anaphylaxis like acute severe urticaria and facial angioedema together with nausea, vomiting, dyspnea, wheezing, chest tightness, and hoarseness after ingestion of vegetable sandwiches containing mayonnaise and mustard. Also, the serum levels of specific IgE against mustard were found to be greatly enhanced (23.6 kU/I) [43]. Another case study from Spain reported that ingestion of small amount of mustard sauce resulted into anaphylaxis in two individuals. A positive SPT reaction against mustard was evident in both persons [44]. Further, Domínguez et al. isolated allergen from mustard seed extracts by gel filtration and this allergen inhibited more than 60% of the binding of mustard-specific IgE from a pool of seven patients’ sera. The allergenic activity was also confirmed by histamine release from whole blood from two sensitive patients [45].

Canada is the second largest producer and largest exporter (75–80%) of mustard, while the province of Saskatchewan produces 80% Canadian mustard [46]. Mustard is included in the list of priority food allergens in Canada since 2012. In Canada, mustard allergy is also very prevalent as it is very much utilized in various foods and food preparations [43].

India is also one of the largest consumers of mustard in the form of whole seeds, mustard powder, and mustard oil but hardly any data with regard to mustard allergy is available from India. A study conducted by Tripathi et al. (2001) revealed that out of 78 patients, 78% of subjects showed positive reaction to mustard allergen extract and 74% were SPT positive for mustard family pollen extract that suggest strong cross-reactivity [47]. According to EuroPrevall-INCO survey (a project that has been developed to evaluate the occurrence of food allergies in China, India, and Russia using the standardized methodology of the EuroPrevall protocol used for studies in the European Union), mustard is listed in 24 priority food allergens [48]. In a study, 1860 patients (12–62 years) were screened for food allergy using standard questionnaire and SPT. Eleven patients showed increased IgE levels against mustard [49]. Further, Singh et al. carried out SPT on 96 patients with history of sensitization to inhalant or food allergens with genetically modified (expressing choline oxidase (codA) gene from Arthrobacter globiformis) and native B. juncea leaf extracts. Of these, six patients were SPT positive to GM and native mustard extracts and ten patients showed 2.5–3 times higher specific IgE levels to GM and native mustard in comparison to negative controls [50].

Further, a case report from Italy revealed that one patient was admitted twice in a year after ingestion of pizza due to symptoms like acute urticaria with edema of the glottis. The patient was SPT positive for the Brassicaceae family (mustard, turnip, cabbage, and cauliflower). When the patient was injected with mustard antigen extracted from seeds of yellow and black mustard, an intense wheal and flare reaction was observed with shock and edema of glottis [28].

A 40-year-old Swedish woman who was working as a cook had eczema with profuse vesicular lesions. Prick test using crushed white and black mustard seeds in this woman showed positive results with marked wheal and flare reaction. She was also SPT positive for rape seed (B. napus) and was advised to avoid eating foods containing mustard and rape seed [51]. Two case reports of mustard allergy were also revealed from the study conducted by Gaig et al. In the first case, a 25-year-old woman showed urticaria with pertusis after ingestion of pomegranate. She was allergic to peach, grapes, banana, and mustard. In another case, a 3-year-old girl showed urticaria, facial angioedema, and experienced shortness of breath after eating pomegranate. SPT results were positive to mites, mustard, peanut, almonds, fig, peach, corn, and rice [52]. Connors et al. reported a case of mustard allergy wherein, a 50-year-old woman had symptoms of anaphylaxis following ingestion of a hamburger with mustard. She had a previous history of similar symptoms with ingestion of mustard [53].

One case report from USA showed that a 29-year-old man consumed quinoa dish and experienced an episode of facial flushing and angioedema together with urticaria, chest tightness, and vomiting within 30 min of consumption of the dish. Previously, he had consumed and tolerated quinoa without any reaction. The quinoa dish contained chicken, rice, avocado, ginger, onion, potatoes, tomato, nutmeg, yeast, soy, wheat, black pepper, thyme, and mustard seed. Skin prick test was positive to mustard and percutaneous skin testing was positive to soy [54]. Stricker et al. screened 168 patients having idiopathic anaphylaxis with a battery of 79 food-antigen skin prick tests, out of which 3 patients were SPT positive to mustard, and a positive clinical correlation with skin test and anaphylaxis was observed in the case of mustard [55]. A study conducted by Mailhol et al. included a total of 386 children with atopic dermatitis, out of which 69 children were diagnosed with food allergy, and prevalence of mustard allergy was 1% [56]. Furthermore, Asero et al. screened 49 patients monosensitized to LTPs and found that 4% of the people were SPT positive to mustard [57].

A study in Finland was conducted on 50 patients with spice extracts, and their potential to cause skin reactions were analyzed by SPT. Notable, mustard was reported as one of the major spices that cause skin reactions in sensitive individuals [58]. The case reports are summarized in Table 3.

Table 3 Clinical data on mustard allergy in chronological order
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Mustard-Induced Respiratory Allergies

Individuals suffering from food allergy may also show symptoms of respiratory allergy (sneezing, coughing, rhinoconjunctivitis, and sometimes asthma) due to inhalation of a wide variety of plant-derived pollens. Some foods are responsible for inflammation of respiratory tract and cause allergic rhinitis/asthma-like symptoms in susceptible individuals. People suffering from pollinosis are more likely to develop food allergy and 30–60% of patients with pollen allergy in the European population do suffer with plant food allergy [59,60,61]. Little is known about the mustard-induced respiratory allergies as on date, and relatively few studies are available in this regard (Table 3). The explanation of these kinds of allergy can be given by cross-reactivity among plant foods and pollens.

A study conducted by Singh et al. investigated allergenicity of antigenic extracts of pollens of 4 species of Brassica and revealed that out of 159 atopic subjects, 21.4% were positive to at least one or other species of Brassica pollen, with the highest skin positivity (13.2%) to B. campestris extract [62]. Further, a study conducted by Toorenenbergen et al. reported that a patient working in spice factory experienced symptoms of rhinitis and shortness of breath. These symptoms became severe during Monday to Friday and relieved during weekends. It was noticed that due to exposure of spice dust, the person faced such problems [63]. A woman (ex-smoker) with history of rhinitis and asthma was found to be sensitive to yellow mustard allergen Sina2 (11S globulin/legumin) and rapeseeds. It was the first case indicating the involvement of mustard allergen (Sina2)-11S globulin in the etiology of asthma upon inhalation [64].

Moreover, a study conducted by Niinimaki et al. in Finland revealed that 49 patients allergic to birch pollen or mugwort pollen when subjected to SPT with extracts of different commonly used spices (coriander, caraway, cayenne and mustard) showed SPT-positive results with all these spices and mustard is one of the major causes of allergic reactions in these sensitive individuals [65]. A study by Morriset et al. reported that out of 30 subjects, 7 patients were allergic to mustard and 4 individuals (4/7 = 57%) showed symptoms of respiratory allergy, but symptoms were moderately severe [13]. An interesting prospective study in 38 mustard-sensitive patients was performed by Figueroa et al. suggesting the strong relationship between mustard hypersensitivity and mugwort pollen sensitization (97.4% of patients) [42].

Similarly, Yagami et al. demonstrated a case (22-year-old woman) of pollen-food allergy syndrome (PFAS) from melon and latex allergy (LA) to natural rubber latex (NRL) antigen. She was also allergic to birch pollen (Bet v 2), natural rubber latex allergen (Hev b 8), and timothy allergen (Phl p 12), as well as to some common spices including mustard seeds, cumin, fennel, dill, fenugreek, cayenne, ginger, cardamom, garlic, garam masala, and coconut milk with symptoms of atopic dermatitis, allergic rhinitis, and pollinosis [66]. Anguita et al. screened 12 olive orchard workers, who had a history of rhinitis and/or bronchial asthma, with SPT with S. alba pollen extract. The patients were found sensitized to S. alba pollen extract, and also showed a positive nasal challenge test response. Therefore, the authors concluded that S. alba pollen acts as occupational allergen for olive farmers [67].

Mustard-Induced Skin Allergies

Mustard has the ability to provoke respiratory allergies. There are emerging evidences indicating the involvement of mustard in inducing skin allergies. Allylisothiocyanate (a volatile chemical), a major antigen of mustard oil, which is responsible for the pungent odor and flavor, is also capable of inciting contact dermatitis [68, 69]. A study conducted by Gaul et al. showed that a 61 years old Negro man developed symptoms of contact dermatitis on hands, forearms, face, and neck. Additionally, patch test was also found to be positive (8 cm patch) for household liniment that contained clove oil, wintergreen oil, synthetic mustard oil, sassafras oil, peppermint oil, and cajeput oil. Later on, allylisothiocyanate present in synthetic mustard oil was noticed as the main culprit in the liniment as 0.1% of mustard oil produced approximately 2 cm vesicular patch [70]. Similarly, a 34-year-old woman felt burning sensation and erythematous skin after application of compress crushed mustard seeds. Five days later, she ate German sausage having mustard as ingredient that resulted into serious exacerbation with development of bullae. Further, patch tests and prick tests were performed using 0.001 to 0.1% allylisothiocyanate, mustard seeds 1:100 to 1:1, and different commercially available mustard preparation. However, only allergic reaction with superficial necrosis was observed with 5% allylisothiocyanate [71].

In addition to allylisothiocyanate, some proteins from mustard are also responsible for causing allergic reactions in sensitive individuals. Leanizbarrutia et al. reported two cases of mustard allergy with severe anaphylaxis after eating small quantities of prepared mustard. Interestingly, when mustard extract was treated with proteolytic enzymes, its allergenic capacity was lost suggesting that proteins present in mustard extract were responsible for allergic responses [72]. Moreover, a case report showed that a 38-year-old female salad maker has a history of hand dermatitis for 2 years with itchy vesicular eruption on fingers and forearms. It was noticed that being a salad maker, she was continuously exposed (dermal exposure) to salad cream that contain various members of the mustard family. Patch test results showed + 1 reaction against 1% aqueous concentration of black mustard (B. nigra) and + 3 positive for two commercial mustards. In addition, RAST test was found positive for IgE against extract of black mustard [73]. Further, another case of occupational contact dermatitis came into existence when major fish stick factory workers in Northern Norway complained about the irritant reaction on skin, eyes, and upper airways. Some of them developed symptoms of eczema on the hands, arms, and face. Later, it was observed that 3 individuals out of 16 had immediate reaction to mustard and all the 3 persons had the history of atopic disease [74].

Another interesting case revealed that a 48-year-old woman with history of eczema and allergic contact dermatitis developed swelling, itching, and redness on her back after application of Chinese herbal medicine for the treatment of tracheitis. Patch test results showed only positive reaction (erythema and bulla) to white mustard seed [75]. As white mustard seeds are widely used in Chinese herbal medicine, persons with history of allergy should avoid its application. Furthermore, a study from Finland included 64 children with atopic dermatitis. These children were screened for sensitization with turnip rape and oilseed rape. Almost all the children (97%) were sensitized to mustard with total IgE values higher than controls [76]. Poikonenet et al. carried out a study on Finnish (14 children) and French children (14 children) to find out the scenario of turnip rape and mustard allergy among them and also the cross-reactivity between turnip rape and mustard. Oral challenge with turnip rape was positive in 14 (100%) Finnish subjects and 5 (36%) French children whereas mustard challenge was positive in 5 (36%) Finnish and 5 (36%) French children. Moreover, most of the children (both Finnish and French) had serum IgE against oilseed rape and mustard indicating towards the possible cross-reactivity among turnip rape, oilseed rape, and mustard [77].

A 54-year-old female working as a vegetable farmer presented with painful pruritic skin lesions on both hands (eczema). Later on, further experimental analysis using RAST, skin prick, and patch testing confirmed that the woman was allergic to salad mustard (rucola), spinach, and chives [78]. Further, a young woman had recurrent urticaria and angioneurotic edema upon ingestion of mustard and mayonnaise. By RAST, IgE-mediated allergy to mustard and botanically related plants was confirmed [79]. Similarly, Niinimäki et al. did scratch tests with common spices in 1120 atopic and 380 non-atopic patients. Mustard was one of the foods found responsible for the vast majority of the skin reactions [80].

An interesting case from India revealed that topical application of mustard oil caused Pityriasisrosea-like eruptions on skin. A 25-year-old man suffered from multiple, well-demarcated, erythematous, scaly plaques of oval shape of variable sizes, ranging approximately from 0.5 to 3 cm in width and 1 to 5 cm in length. He applied mustard oil for body massage after bathing and after removal of the application, the lesions were cleared. Later examination confirmed that the man was SPT positive to all concentrations of mustard oil [81]. A study conducted by Diamond et al. revealed that nasturtium, a plant that contains mustard oil, causes skin irritation and allergic contact dermatitis [82], (Table 3).

Major Mustard Allergens

Mustard, a widely consumed spice, can elicit IgE-mediated allergic reactions in sensitive individuals. Four major allergens from yellow mustard (S. alba or B. alba) have been identified and characterized namely (a) Sin a 1—2S albumin,14 kDa [83]; (b) Sin a 2—11S globulin, 51 kDa [84]; (c) Sin a 3—lipid transfer protein, 12 kDa [85]; and (d) Sin a 4—profilin, 13–14 kDa [85]. The structure of the major mustard allergens is provided in Fig. 1.

Fig. 1
figure 1

The structural representation of major mustard allergens. a Sin a1. b Bra j1. c Sin a 2. d Sin a 3. e Sin a 4

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Sin a 1 belongs to storage protein of the 2S albumin [86, 87]. 2S albumins is a group of seed storage proteins that are classified on the basis of their sedimentation coefficient [88] and are important for many crucial functions of plants including nutrient source (amino acids and carbon skeletons), germination, seedling growth, and host defense against fungal attack. In the last few decades, various members of this protein family have been identified as major food allergens and were characterized by their IgE-binding ability. Sin a 1 (from S. alba), Bra j 1 (from B. juncea), Ric c 1 (from Ricinus communis), Ber e 1 (from Brazil nut), and Sesi 1 (from Sesamum indicum) are some important allergens that have been identified and classified as 2S albumins.

Sin a 1: 2S albumin fraction of mustard extract, i.e., Sin a 1 has been found to be the most potent allergen of yellow mustard than other fractions [86]. This protein (14.1 kDa) is made up of 2 polypeptides consisting 39 and 88 amino acids respectively that are held together by means of 2 disulfide bonds. Epitope mapping studies of Sin a 1 using ten specific monoclonal antibodies (mAbs) revealed that this allergen contains two immunodominant regions to which all these mAbs were directed, and binding of mAbs to these regions significantly reduced the IgE binding to Sin a 1. In addition, one out of ten mAbs recognizes a continuous epitope on a large chain of protein that is also a characteristic of IgE-binding epitope. Only one tyrosine residue within this epitope is thought to be the immuno-dominant portion of the allergen [87].

Sin a 1 is considered as the most suitable diagnostic marker to identify the prevalence and sensitization of mustard as levels of specific IgE against mustard were almost comparable to that of specific IgE levels against Sin a 1 alone, suggesting that it is a major allergen of S. alba. Sin a 1 can interact with membrane lipids that enhance its uptake at the intestinal barrier and thereby increases the resistance of this allergen against protease digestion. Interestingly, it was also noticed that binding of Sin a 1 to B cells results into their activation via cross-linking of cell surface proteins that leads to humoral immune response [89].

Sin a 2: Sin a 2 was identified as the second major allergen from yellow mustard seeds using two-dimensional gel electrophoresis, mass spectrometry, and Edman degradation that belong to the 11S globulin family [90]. 11S globulins are salt-soluble proteins belonging to the family of seed storage proteins (Cupin superfamily). Sin a 2 (51 kDa) is composed of two polypeptides of 36 and 23 kDa linked by a disulfide bond. These two chains can be separated by treatment of reducing agents, but they retain their IgE-binding capacity even after separation [90]. In Sin a 2, the highly conserved asparaginyl cleavage site (characteristic of 11S globulin family) is located between positions Asn297and Gly298. In addition, out of five cysteine residues of Sin a 2, four are conserved in all allergenic 11S globulins. The major difference between Sin a 2 and other allergens of 11S globulin family is that the former contains a region enriched in Gln and Gly (positions 109 to 147) [84]. Sin a 2 is used as a marker to predict severity of symptoms. ELISA inhibition experiment with Sin a 1 and Sin a 2 revealed that incubation of Sin a1 with mustard-sensitive patients resulted into reduction of IgE binding (66.5% average inhibition) to mustard seed extract whereas preincubation of the patient’s sera with Sin a 2 also resulted into reduced IgE binding (average 36.5% inhibition) to mustard seed extract. But, when sera were incubated with mixture of both the allergens, IgE binding to whole extract was abolished nearly by 90% [90].

Sin a 3 and Sin a 4: Other two allergens, Sin a 3 and Sin a 4, are present in very low amounts in yellow mustard seeds and belong to different families of panallergens. Sin a 3 belongs to non-specific lipid transfer protein (nsLTP) family that consists of a single chain of 92 amino acids with MW 12.3 kDa. Gastric and intestinal digestion of Sin a 3 resulted into partial proteolysis and the digested low-molecular-weight (MW) band retained IgE-binding ability up to 2 h [91].

Sin a 4 belongs to profilin plant family that contains a single chain of 14.2 kDa with 131 amino acids. Profilins from different plant species share highly conserved sequences and are major panallergens [92]. Gastric digestion of Sin a 4 resulted into approximately 80% digestion, but it retained its IgE-binding ability (20% residual activity) after 1 h of digestion. In contrast, intestinal digestion of Sin a 4 abolished its IgE reactivity [91]. When theoretical cleavage sites for pepsin were detected for mustard allergens using in silico approaches, it has been noticed that Sin a 4 has higher number of cleavage sites at the surface than Sin a 1 and Sin a 3 that make the former more liable to digestion than the latter ones. Theoretical cleavage sites for trypsin and chymotrypsin (n = 14 + 8) in Sin a 1 were more than pepsin (n = 17). In Sin a 3, all the cleavage sites for trypsin were located on the surface whereas only 55% sites for chymotrypsin were found on the surface of allergen. Sin a 4 displays cleavage sites inside and outside of structure for both the enzymes [91]. These results indicate that all mustard allergens are more prone to intestinal digestion rather than gastric digestion.

Bra j 1: Bra j 1, a major allergen from oriental mustard (B. juncea), consists of two polypeptides of 37 and 92 amino acids respectively linked by disulfide bonds. Bra j 1 is a member of 2S albumin family and is closely related to Sin a 1 from yellow mustard. González et al. reported that Bra j 1 was recognized by sera of mustard-sensitive patients. Moreover, five different fractions from B. juncea with MW in the range from 16 to 16.4 kDa were separated using ion exchange chromatography and observed that all these isoallergenic forms of Bra j 1 gave a single band using Sin a I-specific rabbit polyclonal serum [93]. These five isoallergenic forms named as Bra j 1A, Bra j 1B, Bra j 1C, Bra j 1D, and Bra j 1E [94] were recognized by monoclonal antibody 2B3 raised against Sin a 1. In addition, the sequence of Bra j 1 when aligned with Sin a 1 showed 89% sequence similarity [95]. The details of mustard allergens are provided in Table 4.

Table 4 Major mustard allergens
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Cross-Reactivity of Mustard Allergens with Other Allergens

People allergic to a specific food may also react to other food/environmental allergens due to the presence of homologous epitopes that may confuse the immune system and make it react against the allergen other than what they are allergic to. There are chances that mustard allergens show cross-reactivity to other members of Brassicaceae family and other plant families; therefore, it is important to understand the scenario of cross-reactivity of mustard with allergens of other related crops. However, little is known about the cross-reactivity between mustard seeds and leafy vegetables of Brassicaceae family but it has been shown that mustard did not show any cross-sensitivity with vegetables of the same family [96]. However, one case report suggested the involvement of broccoli (belongs to Brassicaceae family) in cross-reactivity with mustard. A 73-year-old Japanese male experienced dyspnea and swelling of the lips and eyelids after consumption of boiled broccoli. The person was SPT positive for raw broccoli (+ 4), heated broccoli (+ 4), mustard (3+), and heated mustard (+ 3) and therefore was advised to avoid consuming broccoli and mustard [97]. Moreover, the individuals who are allergic to one variety of mustard have the great probability of being sensitive to other varieties as well because there are structural similarities among allergens. The characterization of Sin a 1 and Bra j 1 revealed homologous epitopes between both allergens [84, 93, 95]; thus, the people allergic to yellow mustard are also sensitive for oriental mustard. In addition, Sin a 1 also shows a marked cross-reactivity with rape seed allergen Bn III [98]. Further, 2S albumins of 5 Brassica species (B. campestris, B. oleracea, B. nigra, B. juncea, and B. carinata) display more than 85% homology and significant similarity with some other 2S albumin fractions of Crucifereae family [99]. A study conducted by Asero et al. showed that 2S albumin from sunflower seed is cross-reactive to mustard 2S albumin, and therefore, it is necessary to carry out SPT for mustard in sunflower-sensitive patients [100]. Similarly, amino acid sequence of 2S albumin from R. communis showed the similarity with other 2S albumins including mustard (both to Bra j 1 and Sin a 1) and may cause cross-sensitization within the population [101]. Similarly, cross-reactivity between rapeseed allergen BnIII and Sin a 1 was also observed as Sin a 1 was recognized by the IgE antibodies present in the serum of the rapeseed allergic patient [102].

Many allergens from the 11S protein family have been identified and characterized including Ara h 3 from peanut [103], Lup a 11S globulin from Lupin [104], Cic a 11S globulin from chickpea [105], Cor a 9 from hazelnut [106], glycinins G1-G2 from soybean [107], Ber e 2 from Brazil nut [108], and Ana o 2 from cashew nut [109]. Some studies revealed that 11S globulins may play an important role in the cross-reactivity between coconut and walnut [110], among buckwheat, poppy, and hazelnut [111] and between peanut and different seeds spices. Sin a 2 from mustard belongs to 11S globulin family of proteins and shares IgG epitopes with allergenic 11S globulins from tree nuts (almond, hazelnut, pistachio, and walnut), but not from peanut. The IgG binding of Sin a 2 was reduced when sera of mustard-sensitive patients were incubated with almond extract (77% inhibition), walnut extract (60% inhibition), hazelnut extract (53% inhibition), and pistachio extract (43% inhibition). Similarly, the IgE-binding ability of Sin a 2 was also reduced by incubating the mustard-sensitive patient’s sera with almond extract (63% inhibition), walnut extract (49% inhibition), pistachio extract (34% inhibition), peanut (32% inhibition), and hazelnut (15% inhibition). These research findings indicate that Sin a 2 is sensitizing allergen of mustard that shares conserved IgE epitopes with other allergenic 11S globulins that play an important role in the cross-reactivity among mustard, tree nuts and peanut.

The patients that had Sin a 2-specific IgE in their sera also showed positive specific IgE to the cross-reactive mustard allergens (panallergens) Sin a 3 and to Sin a 4 [112]. Further, it has also been found that people with mustard allergy also show symptoms against some pollens like birch, mugwort, and ragweed. Some researchers believe that mustard allergy is strongly associated with mugwort allergy called as mustard-mugwort allergy syndrome. Figueroa et al. conducted a study on 38 Spanish patients and reported that about 97.4% of patients with mustard allergy were also allergic to mugwort pollen with partial cross-reactivity confirmed by inhibition assay. In addition, all patients were also allergic to other members of Brassicaceae family, nuts (97.4%), leguminous crops (94.7%), corn (78.9%), and Rosaceae fruit (89.5%) that may be explained by cross-reactivity among allergens [42]. The potential cross-reactivity between mugwort and mustard may be due to LTPs (Art v 3, Sin a 3) and profilins (Art v 4, Sin a 4) [113]. Both Sin a 3 and Sin a 4 are associated with cross-reactivity with many pollens and plant-derived foods of Rosaceae family [114].

Sin a 3 shows 65% similarity with N-terminal sequence of Bra o 3 (nsLTP from cabbage), 50–55% similarity with Pru p 3 (nsLTP from peach), Mal d 3 (nsLTP from apple), Prua v 3 (nsLTP from cherry), Fra a 3 (pathogenesis-related PR-10 protein), and Cor a 8 (nsLTP from hazelnut). Substantial evidence now exists to indicate that the IgE-binding ability of Sin a 3 with sera of mustard-sensitive patients is reduced by both peach pulp and peel extracts [85, 115, 116]. Therefore, it may be possible that Sin a 3 is a key allergen responsible for the cross-reactivity of mustard extract with other allergens. Another mustard allergen Sin a 4 shows around 80% similarity with Cuc m 2 (profilin from melon) and Pru p 4 (profilin from peach), and the cross-reactivity is confirmed by ELISA inhibition as the IgE-binding capacity of Sin a 4 to mustard-sensitive patients’ sera is inhibited by preincubation with melon extract. Moreover, it has been found that Sin a 4 (profilin) has sequence similarity with other profilins including Che a 2 from chenopod, Bet v 2 from birch, and Cuc m 2 from melon. Additionally, Sin a 4-specific antiserum is reactive to these profilins suggesting its involvement in cross-reactivity [117].

Mustard as a Hidden Allergen

Although mustard allergy is not as prevalent as other food allergies and affect a small population, symptoms and severity caused by mustard allergens have been demonstrated by many published reports [14, 29, 41, 44, 96, 118], suggesting that mustard may cause severe anaphylactic reactions that need urgent medical treatment. Mustard may be present as a hidden ingredient in many prepared or prepackaged foods (Table 1) as it is widely used in the food formulations for its pungency, thickening, and stabilizing abilities. Hidden allergens or masked allergens are present in such tiny amounts that sometimes it may be undetectable and cause mild to severe reactions in the previously sensitized individuals. Some allergenic reactions may occur due to the presence of food ingredients consumed in very small amounts (flavorings, honey, cinnamon, lupin, grass pea). The flavorings include mustard, paprika, ginger, oregano, and especially garlic as hidden allergens that sometimes can result in serious reactions. Mustard has also been found in sauces and pizza as hidden allergen [38]. Allergic reactions due to the ingestion of uncommon foods including mustard are usually overlooked as they are very difficult to identify and diagnose [119]. A major problem is how to avoid hidden allergens in the commercial or packaged foods, as reactions occur after ingesting the foods accidently or unknowingly that are responsible for the allergic symptoms. The presence of hidden allergens in foods is not always intentional; their presence is due to many reasons like misleading labels, contamination of foods with allergenic ingredients, carelessness, and listing of ingredients using uncommon term [119].

The Effect of Food Processing on Mustard Allergens

It is well established that protein function (including enzymatic activity), stability (including resistance to proteolytic digestion), and glycosylation patterns may have significant impact on both immunogenicity and hence on the allergenic potential of that particular protein [120]. However, in the case of food allergy, the impact of food processing and food matrix should also be considered while looking towards the allergic potential. Food processing techniques may suppress food allergic reactions by destroying some epitopes but do not completely abolish the allergenic potential of allergens. The type of processing methods also affects the allergenic properties such as roasting reduces the allergenicity of most of the foods but in case of peanut, it results into enhanced allergenicity. Some common processing techniques include heating (thermal processing), enzymatic hydrolysis, acid hydrolysis, physical treatments (such as high-pressure processing or extrusion), the use of preservatives, changes in pH, or combination of two or more of these [121, 122].

At present, no effective preventive treatments exist for mustard allergy or for other food allergies. Hence, susceptible individuals are advised to strictly avoid mustard as whole or mustard-containing foods within their diet [6, 123]. However, novel alternate strategies need to be studied, including use of different thermal processing methods (roasting, autoclaving, boiling, and acid digestion). Most of the mustard allergens are heat resistant. A study demonstrated that Bra j 1, a major allergen from oriental mustard, is denatured at 82 °C [124]. Heat resistance of allergens may be explained by their interaction with other food matrix constituents and thereby forming a more stable structure [95] as well as by disulfide bonds that contribute to form more stable structures. In addition, allergic proteins from mustard remain intact after gastric and intestinal digestion up to 60 min of digestibility period (pH 2 with pepsin and pH 6.8 with pancreatin, respectively) [125]. However, a combination of physical and thermal treatment (e.g., extrusion) may be beneficial in suppressing the allergenicity of mustard seed allergens. Glycosylation and glycation of mustard allergens may also play an important role in the alleviation of mustard-induced allergy. Microbial fermentation and enzymatic or acid hydrolysis can also have the potential to reduce the allergenicity to such an extent that reactions will not be elicited.

Conclusions

A number of reports are available on mustard hypersensitivity in the literature, but there no systematic review on mustard allergy and its major allergens. The current review provides the most relevant clinical studies on mustard allergy, the immunological and biochemical characteristics of mustard allergens that have been accumulated till date. The structural similarity of major mustard allergens with other allergenic proteins that may be involved in cross-reactive reactions, and the different processing techniques that have the potential to reduce the allergenic potential of different mustard allergens have been elucidated in this review. The better understanding of mustard-induced allergic manifestations and exploring the underlying mechanism will be helpful to develop therapeutic options.

Importantly, the current review may be beneficial for clinicians and researchers regarding the better understanding of mustard allergy and to know the probable cross-reactivity of mustard allergens with other allergens from same or different plant families. This will help the allergist and gastroenterologist to establish easy and early diagnosis of mustard allergy especially in the case of idiopathic anaphylaxis for the safety of the susceptible individuals. As avoidance of allergic food is considered the best treatment in the case of allergy, detailed understanding of mustard allergens and the effect of different processing techniques on these mustard allergens will reduce the incidence of mustard-induced allergic manifestations and thereby reduce the length of hospital stay. The information provided in the current review may pave the way to develop more general approaches and immunomodulatory therapies by allergist and gastroenterologist to treat mustard allergy that would be advantageous particularly for those patients who are allergic to multiple allergens. Conclusively, the understanding of mustard allergy will be useful to improve clinical management and quality of life of patients.

The probable underlying mechanism induced by mustard allergens is summarized in Fig. 2.

Fig. 2
figure 2

The probable mechanism of mustard allergen-induced allergic manifestations in different organs that may lead to asthma, allergic rhinitis, atopic dermatitis, and other associated allergic disorders.

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