Proteomic approaches for identifying new allergens and diagnosing allergic diseases

Detection of Allergenic Proteins in Foodstuffs: Advantages of the Innovative Multiplex Allergen Microarray-Based Immunoassay Compared to Conventional Methods

Several factors can affect the allergen content and profile of a specific food, including processing procedures often leading to a decrease in allergenicity, although no change, or even an increase, have also been reported. Evaluation of the effectiveness of a processing procedure requires the availability of reliable methodologies to assess the variation in molecules able to induce allergic reactions in the analyzed food. Conventional and innovative strategies and methodologies can be exploited to identify allergenic proteins in foodstuffs. However, depending on the specific purposes, different methods can be used. In this review, we have critically reviewed the advantages of an innovative method, the multiplex allergen microarray-based immunoassay, in the detection of allergens in foodstuffs. In particular, we have analyzed some studies reporting the exploitation of an IgE-binding inhibition assay on multiplex allergen biochips, which has not yet been reviewed in the available literature. Unlike the others, this methodology enables the identification of many allergenic proteins, some of which are still unknown, which are recognized by IgE from allergic patients, with a single test. The examined literature suggests that the inhibition test associated with the multiplex allergen immunoassay is a promising methodology exploitable for the detection of IgE-binding proteins in food samples.

Identification and Characterization of Glycoproteins and Their Responsive Patterns upon Ethylene Stimulation in the Rubber Latex

Natural rubber is an important industrial material, which is obtained from the only commercially cultivated rubber tree, Hevea brasiliensis. In rubber latex production, ethylene has been extensively used as a stimulant. Recent research showed that post-translational modifications (PTMs) of latex proteins, such as phosphorylation, glycosylation and ubiquitination, are crucial in natural rubber biosynthesis. In this study, comparative proteomics was performed to identify the glycosylated proteins in rubber latex treated with ethylene for different days. Combined with Pro-Q Glycoprotein gel staining and mass spectrometry techniques, we provided the first visual profiling of glycoproteomics of rubber latex and finally identified 144 glycosylated protein species, including 65 differentially accumulated proteins (DAPs) after treating with ethylene for three and/or five days. Gene Ontology (GO) functional annotation showed that these ethylene-responsive glycoproteins are mainly involved in cell parts, membrane components and metabolism. Pathway analysis demonstrated that these glycosylated rubber latex proteins are mainly involved in carbohydrate metabolism, energy metabolism, degradation function and cellular processes in rubber latex metabolism. Protein-protein interaction analysis revealed that these DAPs are mainly centered on acetyl-CoA acetyltransferase and hydroxymethylglutaryl-CoA synthase (HMGS) in the mevalonate pathway for natural rubber biosynthesis. In our glycoproteomics, three protein isoforms of HMGS2 were identified from rubber latex, and only one HMGS2 isoform was sharply increased in rubber latex by ethylene treatment for five days. Furthermore, the HbHMGS2 gene was over-expressed in a model rubber-producing grass Taraxacum Kok-saghyz and rubber content in the roots of transgenic rubber grass was significantly increased over that in the wild type plant, indicating HMGS2 is the key component for natural rubber production.

Effects of ohmic heating on the immunoreactivity of β-lactoglobulin – a relationship towards structural aspects

Ohmic heating changes the immunoreactivity of monomeric and aggregated β-LG forms.

Characterization of IgE-binding proteins in the salivary glands of Simulium nigrogilvum (Diptera: Simuliidae)

Simulium dermatitis is an IgE-mediated skin reaction in animals and humans caused by the bites of black flies. Although Simulium nigrogilvum has been incriminated as the main human-biting black fly species in Thailand, information on its salivary allergens is lacking. Salivary gland extract of S. nigrogilvum females was subjected to sodium dodecylsulfate-polyacrylamide gel electrophoresis, and the separated components were applied onto nitrocellulose membranes for immunoblotting, which was performed by probing the protein blots with sera from 17 individuals who were allergic to the bites of S. nigrogilvum. IgE-reactive protein bands were characterized further by liquid chromatography-mass spectrometry (LC-MS/MS) analysis. Nine protein bands (79, 42, 32, 25, 24, 22, 15, 13, and 11 kDa) were recognized in the serum of the subjects. Four of the nine protein bands (32, 24, 15, and 11 kDa) showed IgE reactivity in all (100%) of the tested sera, and they were identified as salivary secreted antigen 5-related protein, salivary serine protease, erythema protein, and hypothetical secreted protein, respectively. Three other proteins, salivary serine protease (25 kDa), salivary D7 secreted protein (22 kDa), and hypothetical protein (13 kDa), reacted with > 50% of the sera. The relevance of the identified protein bands as allergens needs to be confirmed by using pure recombinant proteins, either in the in vivo skin prick test or in vitro detection of the specific IgE in the serum samples of allergic subjects. This will be useful for the rational design of component-resolved diagnosis and allergen immunotherapy for the allergy mediated by the bites of black flies.

Consideration of methods for identifying mite allergens

House dust mites are small arthropods that produce proteins—found in their feces, body parts, and eggs—that are major triggers of human allergies worldwide. The goal of this review is to describe the current methods used to identify these allergens. A literature search for allergen identification methods employed between 1995 and 2016 revealed multiple techniques that can be broadly grouped into discovery and confirmation phases. The discovery phase employs screening for mite proteins that can bind IgEs in sera from animals or patients allergic to dust mites. The confirmation phase employs biochemical methods to isolate either native or recombinant mite proteins, confirms the IgE binding of the purified allergens, and uses either in vitro or in vivo assays to demonstrate that the purified antigen can stimulate an immune response. The methods used in the two phases are defined and their strengths and weaknesses are discussed. The majority of HDM-allergic patients may respond to just a small subset of proteins, but new protein discovery methods are still warranted in order to develop a complete panel of HDM allergens for component resolved diagnosis and patient-tailored therapies.

Identification of Immunoglobulin E-Binding Proteins of the Xerophilic Fungus Aspergillus penicillioides Crude Mycelial Mat Extract and Serological Reactivity Assessment in Subjects with Different Allergen Reactivity Profiles

BACKGROUND

Aspergillus penicillioides is a very common indoor xerophilic fungus and potential causative agent of respiratory conditions. Although people are constantly exposed to A. penicillioides, no proteins with allergenic potential have been described. Therefore, we aim to confirm allergic sensitization to A. penicillioides through reactivity in serological assays and detect immunoglobulin E (IgE)-binding proteins.

METHODS

In an indirect ELISA, we compared the serological reactivity to A. penicillioides between subjects with specific IgE (sIgE) (group 1, n = 54) and no sIgE reactivity (group 2, n = 15) against commercial allergens. Correlations and principal component analysis were performed to identify associations between reactivity to commercial allergens and A. penicillioides. IgE-binding proteins in A. penicillioides were visualized using Western blotting (WB) in group 1. The IgE-binding proteins with the highest reactivity were analyzed by mass spectrometry and confirmed by transcript matching.

RESULTS

There was no statistical significance (p = 0.1656) between the study groups in serological reactivity. Correlations between reactivity to A. penicillioides, dog epithelia, Aspergillus fumigatus, and Penicillium chrysogenum were observed. WB experiments showed 6 IgE-binding proteins with molecular weights ranging from 45 to 145 kDa. Proteins of 108, 83, and 56 kDa showed higher reactivity. Mass spectrometry analysis of these 3 proteins led to the putative identification of NADP-specific glutamate dehydrogenase and catalase B. This was confirmed with transcriptome analysis.

CONCLUSIONS

These results provide evidence of the presence of potential allergenic components in A. penicillioides. Further analysis of the putatively identified proteins should reveal their allergenic potential.

Component-resolved diagnostics in vernal conjunctivitis

BACKGROUND

Conventional diagnostic tests in allergy are insufficient to clarify the origin of vernal conjunctivitis (VC).

OBJECTIVES

To evaluate IgE-mediated hypersensitivity by component-resolved diagnosis (CRD) in tears and serum from patients with VC and to evaluate how to treat patients with identified triggering allergens by specific immunotherapy.

METHODS

Patients were divided into 3 groups: (1) patients with VC (25 patients), (2) patients allergic to grass pollen with seasonal allergic conjunctivitis (AC) (50 patients), and (3) healthy blood donors (50 patients). If triggering allergens were detected, specific conventional immunotherapy was administered for 1 year.

RESULTS

Twenty-five patients with VC were evaluated. The identified triggering allergens were n Lol p 1 (11 patients), n Cyn d 1 (8 patients), group 4 and 6 grasses (6 patients), and group 5 grasses (5 patients). Prick test and pollen IgE test results were positive in one patient. Clinical improvement was observed in 13 of the 25 patients with VC after 1 year of specific immunotherapy.

CONCLUSION

CRD seems to be a more sensitive diagnostic tool compared with prick test and IgE detection. Specific CRD-led immunotherapy may achieve clinical improvements in patients with VC.

Proteomics of cypress pollen allergens using double and triple one-dimensional electrophoresis

Italian cypress (Cupressus sempervirens, Cups) pollen causes allergic diseases in inhabitants of many of the cities surrounding the Mediterranean basin. However, allergens of Cups pollen are still poorly known. We introduce here a novel proteomic approach based on double one-dimensional gel electrophoresis (D1-DE) as an alternative to the 2-DE immunoblot, for the specific IgE screening of allergenic proteins from pollen extracts. The sequential one-dimensional combination of IEF and SDS-PAGE associated with IgE immunoblotting allows a versatile multiplexed immunochemical analysis of selected groups of allergens by converting a single protein spot into an extended protein band. Moreover, the method appears to be valuable for MS/MS identification, without protein purification, of a new Cups pollen allergen at 43 kDa. D1-DE immunoblotting revealed that the prevalence of IgE sensitization to this allergen belonging to the polygalacturonase (PG) family was 70% in tested French allergic patients. In subsequent triple one-dimensional gel electrophoresis, the Cups pollen PG was shown to promote lectin-based protein-protein interactions. Therefore, D1-DE could be used in routine work as a convenient alternative to 2-DE immunoblotting for the simultaneous screening of allergenic components under identical experimental conditions, thereby saving considerable amounts of sera and allergen extracts.

Identification of the major allergen of Macrobrachium rosenbergii (giant freshwater prawn)

OBJECTIVE

To characterize the major allergens of Macrobrachium rosenbergii (giant freshwater prawn).

METHODS

Raw and cooked extracts of the giant freshwater prawn were prepared. The IgE reactivity pattern was identified by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting technique with the sera of 20 skin prick test (SPT) positive patients. The major allergen identified was then characterized using the proteomics approach involving a combination of two-dimensional (2-DE) electrophoresis, mass spectrometry and bioinformatics tools.

RESULTS

SDS-PAGE of the raw extract showed 23 protein bands (15-250 kDa) but those ranging from 40 to 100 kDa were not found in the cooked extract. From immunoblotting experiments, raw and cooked extracts demonstrated 11 and 5 IgE-binding proteins, respectively, with a molecular mass ranging from 15 to 155 kDa. A heat-resistant 36 kDa protein was identified as the major allergen of both extracts. In addition, a 42 kDa heat-sensitive protein was shown to be a major allergen of the raw extract. The 2-DE gel fractionated the prawn proteins to more than 50 different protein spots. Of these, 10 spots showed specific IgE reactivity with patients' sera. Matrix assisted laser desorption/ionization-time of flight (MALDI-TOF) analysis led to identification of 2 important allergens, tropomyosin and arginine kinase.

CONCLUSIONS

It can be concluded that the availability of such allergens would help in component-based diagnosis and therapy of prawn allergies.

Proteomic analysis in allergy and intolerance to wheat products

Owing to its extensive use in the human diet, wheat is among the most common causes of food-related allergies and intolerances. Allergies to wheat are provoked by ingestion, inhalation or contact with either the soluble or the insoluble gluten proteins in wheat. Gluten proteins, and particularly the gliadin fraction, are also the main factor triggering celiac disease, a common enteropathy induced by ingestion of wheat gluten proteins and related prolamins from oat, rye and barley in genetically susceptible individuals. The role of gliadin and of its derived peptides in eliciting the adverse reactions in celiac disease are still far from being completely explained. Owing to its unique pathogenesis, celiac disease is widely investigated as a model immunogenetic disorder. The structural characterization of the injuring agents, the gluten proteins, assumes a particular significance in order to deepen the understanding of the events that trigger this and similar diseases at the molecular level. Recent developments in proteomics have provided an important contribution to the understanding of several basic aspects of wheat protein-related diseases. These include: the identification of gluten fractions and derived peptides involved in wheat allergy and intolerance, including celiac disease, and the elucidation of their mechanism of toxicity; the development and validation of sensitive and specific methods for detecting trace amounts of gluten proteins in gluten-free foods for intolerant patients; and the formulation of completely new substitute foods and ingredients to replace the gluten-based ones. In this article, the main aspects of current and prospective applications of mass spectrometry and proteomic technologies to the structural characterization of gluten proteins and derived peptides are critically presented, with a focus on issues related to their detection, identification and quantification, which are relevant to the biochemical, immunological and toxicological aspects of wheat intolerance.

Identification of allergens homologous to Alt a 1 from Stemphylium botryosum and Ulocladium botrytis

Several studies have demonstrated that proteins homologous to the Alt a 1 major allergen of Alternaria alternata are expressed in other members of the Pleosporaceae family. However, since no direct biochemical data have been reported concerning the presence of Alt a 1 allergen homologues capable of binding IgE in the excretion-secretion products of Stemphylium and Ulocladium, our objective was to explore their presence in Stemphylium botryosum and Ulocladium botrytis. S. botryosum and U. botrytis culture filtrate extracts were analyzed by two-dimensional (2D)-electrophoresis and 2D-immunoblotting using polyclonal rabbit antibodies raised against recombinant Alt a 1, as well as five human sera from patients allergic to Alternaria. Cross-reactivity immunoassays were performed by ImmunoCAP inhibition and 2D-immunoblotting inhibition. IgE-binding proteins recognized by the rabbit antiserum raised against Alt a 1, with apparent molecular weights of 17-18 kDa and isoelectric points of 4, were identified as Alt a 1-like proteins. Alt a 1 inhibited IgE-specific binding to the Alt a 1 homologues from S. botryosum and U. botrytis. In conclusion, it was demonstrated that allergens which are homologous to Alt a 1 are expressed in the excretory-secretory materials of the phylogenetically-related species S. botryosum and U. botrytis.

Proteomic analysis in the identification of allergenic molecules

Conventional and innovative strategies can be exploited to identify and characterize new allergenic proteins. With the aim of obtaining suggestions for future improvements, this article describes our attempt to understand and describe some of the advantages and pitfalls of the methodologies and procedures often used in this field. The analysis includes the protein extract preparation, starting from the allergenic source, the separation of the proteins contained in a mixture and the detection, identification and characterization of IgE-binding molecules. Classic and emerging proteomic technologies, including mass spectrometry-based methodologies, Edman degradation procedure, microarray-based techniques and bioinformatics search strategies, have been explored. A comparative analysis of biochemistry-based proteomics and molecular biology strategies has also been given.

Authentication of food allergen quality by physicochemical and immunological methods

Purified allergens are required to detect cross-contamination with other allergenic foods and to understand allergen interaction with other components of the food matrix. Pure allergens are also used for the diagnosis and treatment of food allergies. For example, serological methods are being developed to improve the quality of diagnosis, and to reduce the need for food challenge tests. In addition, recombinant allergens are being evaluated as candidate vaccines for safe and efficacious specific immunotherapy. Pure allergens are indispensable as reference materials for the calibration and standardization of methods between different laboratories and operators for risk assessment in the food industry. Therefore, there is a need for well-defined purified food allergens. In this context, a panel of 46 food allergens from plant and animal sources has been purified, from either the food sources or as recombinant forms, within the EU-funded EuroPrevall project. These allergens have been characterized by a battery of diagnostic tests demonstrating that they constitute an authentic, well-defined library of comparable quality. The review summarizes the applications, potentials and limitations of key techniques used for the characterization and authentication of these allergen preparations, with a special emphasis on protein purity and identity, folding, post-translational modifications and immunochemical properties. One key area identified is the development of powerful analytical techniques, such as mass spectrometry and nuclear magnetic resonance, to improve the authentication of allergens for routine applications in allergy management.

Two-dimensional electrophoresis and IgE-mediated food allergy

Food allergy is recognized as one of the major health concerns. It is estimated that ca. 4% of the population is affected by food allergenic disorders. Food allergies are defined as IgE-mediated hypersensitivity reactions. Foods such as peanuts, tree nuts, wheat, soy, cow's milk, egg, fish and shellfish are regarded as responsible for the majority of reactions. The ubiquitous presence of allergens in the human foods coupled with an increased awareness of food allergies warrants to undertake appropriate preventive measures for protecting sensitive consumers from unwanted exposure to offending food allergens. 2-DE followed by immunoblotting and identification of IgE-reactive proteins, as a proteomic approach to identify new allergens in foods, are reviewed. Specific examples of identification of allergens in foods and beverages by using 2-DE and IgE are described. Protein profiling using 2-DE and allergens detection by IgE has become a powerful method for analyzing changes of allergens content in complex matrix during food processing.

IgE sensitization to fungi mirrors fungal phylogenetic systematics

BACKGROUND

Fungal allergy is an elusive disease, and little progress has been made in this field during recent years. Moreover, because of the complexity of the organisms, it is difficult to categorize fungi systematically on the basis of morphologic characterization. However, recent molecular phylogenetics studies have substantially improved fungal categorization. In parallel, new approaches to analyze large IgE antibody datasets enable identification and visualization of IgE sensitization patterns.

OBJECTIVE

To study whether molecular phylogenetic relationships of fungal species, commonly used in allergy diagnosis, also are reflected in IgE sensitization profiles of individuals sensitized to fungi.

METHODS

A dataset was compiled of recorded serum IgE antibody levels to 17 different fungal species from 668 individuals sensitized to at least 1 of the 17 species. By applying a clustering method to this dataset, the fungal species were grouped into a hierarchical organization. Finally, the resulting organization was compared with recently published fungal systematics.

RESULTS

The hierarchical structure of fungi, based on the presence of IgE antibodies in sensitized individuals, very well reflected phylogenetic relationships. Examples include the distinct separation of basal fungi from the subkingdom Dikarya as well as individual cluster formations of fungi belonging to the subphylum Saccharomycotina and order Pleosporales.

CONCLUSION

To our knowledge, this is the first in-depth study that demonstrates a close relationship between molecular fungal systematics and IgE sensitization to fungal species. Because close evolutionary organisms typically have a higher degree of protein similarity, IgE cross-reactivity is likely the main reason for obtained organization.

Evaluation of ash pollen sensitization pattern using proteomic approach with individual sera from allergic patients

BACKGROUND

In Europe, sensitization to ash pollen induces pollinosis with cross-reactivities with other pollen sources. The aim of the study was to identify the repertoire of ash pollen allergens and evaluate the extent of the diversity of the IgE response in ash allergic patients.

METHODS

The IgE reactivities of 114 ash pollen- and eight grass pollen-sensitized patients were screened by 1D immunoblot (SDS-PAGE) against ash pollen extract. The IgE reactivities of 13 ash pollen- and two grass pollen-sensitized patients were then evaluated in 2D immunoblots. Some IgE- and non-IgE-reactive proteins were identified by mass spectrometry.

RESULTS

In 1D analysis, 86% of sera showed binding to Fra e 1 (18-20 kDa), 23% to Fra e 2 (14 kDa), 3% to Fra e 3 (10 kDa) and 57% to High Molecular Weight allergens (HMW, >30 kDa). Individual analysis of 2D immunoblots showed several IgE-binding protein areas among which three were more often recognized: (i) Fra e 1 comprising, at least, 15 isoforms, (ii) a series of acidic spots (45 kDa), and (iii) Fra e 2, the ash profilin. HMW allergens could be resolved in four areas; two unidentified, one homologous to beta-galactosidase and the other to sugar transport proteins. A malate deshydrogenase and calmodulin were shown to be IgE-binding proteins and 10 non-IgE reactive proteins were identified.

CONCLUSIONS

No direct correlation was evidenced between IgE profile and the degree of sensitization even though 2 spectrotypes could be distinguished. Our data contribute to a better delineation of ash pollen allergens and patterns of sensitization.

Novel IgE recognized components of Lolium perenne pollen extract: comparative proteomics evaluation of allergic patients sensitization profiles

In the last years, proteomic investigation provided a powerful tool in molecular characterization of complex allergen sources with relevant implications in both diagnosis and immunotherapic treatment of allergies. We followed a proteomic approach to characterize ryegrass (Lolium perenne) pollen, a common cause of seasonal allergic diseases affecting an increasing part of world population. Peptide shotgun experiments performed on nanoLiquid Ultra Pressure Chromatography coupled with fast Q-TOF MS-MS/MS acquisition protocols (MS(E)) and 2-DE immunoblot combined with MALDI-TOF-TOF analysis allowed the detection of all previously identified ryegrass allergens. Comparative analysis of immunoblot highlighted a class of patients characterized by a more complex 2-DE pattern associated with increased levels of IgE antibodies and by higher susceptibility to multiple sensitization toward different allergen sources. Cluster analysis revealed that all these patients recognized profilin, considered the main cross-reactive allergen in grass pollen. Furthermore, mass spectrometry analysis revealed the presence of other IgE reactive components in ryegrass pollen that might be involved in polysensitization, such as cyclophilin, fructosyltransferase and legumin-like protein.

Proteomic analysis of Parietaria judaica pollen and allergen profiling by an immunoproteomic approach

Parietaria judaica pollen is a common cause of airway allergic disease in the Mediterranean area. Proteome analysis of mature Parietaria judaica pollen by two-dimensional gel electrophoresis (2-DE) and mass spectrometry has established the first reference proteome map of this weed. Proteins involved in a variety of cellular functions as well as the occurrence of allergens were detected. By using 2-DE and immunoblotting with sera from Parietaria judaica allergic patients we obtained a more detailed characterization of Parietaria judaica allergen profile so to improve our comprehension of the pathogenesis of pollen-induced allergic reaction.

Resolution and identification of major peanut allergens using a combination of fluorescence two-dimensional differential gel electrophoresis, Western blotting and Q-TOF mass spectrometry

Peanut allergy is triggered by several proteins known as allergens. In this study, the complexity the peanut allergome is investigated with proteomic tools. The strength of this investigation resides in combining the high-resolving power and reproducibility of fluorescence two-dimensional differential gel electrophoresis with specific immunological detection as well as polypeptide sequencing by high-resolution mass spectrometry. Matching of the peanut proteins in 2D gels was achieved by differential labelling whereby peanut proteins and purified allergens (Ara h 1, Ara h 2 or Ara h 3/4) were run on the same gel. Ten protein spots on a mass line of ca. 63-68 kDa were likely to correspond to Ara h 1. Two doublets on two different mass lines at ca. 16 and 18 kDa matched with purified allergen Ara h 2. The basic and acidic sub-units of Ara h 3/4 were observed at masses of ca. 25 kDa and 40-45 kDa, respectively. Subsequently the antibody-binding capacity of spots corresponding to peanut allergens was investigated by Western blotting of 2D gels using antibodies (IgY) raised against Ara h 1, Ara h 2 and the recombinant 40 kDa sub-unit of Ara h 3/4. Final confirmation of the identity of the protein spots matched after 2D electrophoresis and identified by Western blotting was obtained by in-gel digestion of protein spots and analysis by quadrupole time-of-flight mass spectrometry. By using the method developed in our work, the location and identification of two different isoforms of the allergen Ara h 1, the allergen Ara h 2 and six isoforms of the allergen Ara h 3/4 in 2D peanut protein maps was established.

The property distance index PD predicts peptides that cross-react with IgE antibodies

Similarities in the sequence and structure of allergens can explain clinically observed cross-reactivities. Distinguishing sequences that bind IgE in patient sera can be used to identify potentially allergenic protein sequences and aid in the design of hypo-allergenic proteins. The property distance index PD, incorporated in our Structural Database of Allergenic Proteins (SDAP, http://fermi.utmb.edu/SDAP/), may identify potentially cross-reactive segments of proteins, based on their similarity to known IgE epitopes. We sought to obtain experimental validation of the PD index as a quantitative predictor of IgE cross-reactivity, by designing peptide variants with predetermined PD scores relative to three linear IgE epitopes of Jun a 1, the dominant allergen from mountain cedar pollen. For each of the three epitopes, 60 peptides were designed with increasing PD values (decreasing physicochemical similarity) to the starting sequence. The peptides synthesized on a derivatized cellulose membrane were probed with sera from patients who were allergic to Jun a 1, and the experimental data were interpreted with a PD classification method. Peptides with low PD values relative to a given epitope were more likely to bind IgE from the sera than were those with PD values larger than 6. Control sequences, with PD values between 18 and 20 to all the three epitopes, did not bind patient IgE, thus validating our procedure for identifying negative control peptides. The PD index is a statistically validated method to detect discrete regions of proteins that have a high probability of cross-reacting with IgE from allergic patients.