**1. Introduction**

Type I allergy is an immunoglobulin E (IgE)-mediated chronic disease. As such, disease diagnosis and identification of targeted allergens are primarily based on specific IgE reactivity. Specifically, clinical practices for the diagnosis of allergic disease are most commonly based

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

on skin prick testing [1], which typically involves pricking the skin with a needle or pin containing a small amount of allergen [2]. A second diagnostic test is commonly performed in vitro for allergen-specific immunoglobulin E (IgE), which can accurately evaluate and quantify the presence or absence of IgE specific for the whole allergen extract or single protein components [3].

The importance of IgE in mediating allergic disease, especially immediate-type reactions occurring within minutes of exposure to the allergen, is evident. However, the involvement of allergen-specific T cells and their pathological role in mediating late-phase reactions [4, 5] is often underappreciated. Allergenic proteins are defined based on their ability to bind IgE and the frequency of allergic patients harboring specific IgE antibodies to a given allergen [6, 7]. The potential of an allergenic protein to induce T cell reactivity is mostly not taken into account when classifying a protein as an allergen. Over the past decades, however, the contribution of T cells, specifically T helper 2 (Th2) cells, in mediating the pathogenesis of allergy has been extensively studied [8]. Immunological studies have shown that T cells play a key role early on, before allergic disease is even established. Susceptible individuals initially exposed to allergen mount a dominant Th2 response, resulting in the production of type 2 cytokines, such as IL-4 and IL-13. These cytokines along with a direct physical interaction of T and B cells occurring between CD40L expressed on the surface of the activated T cell and CD40 constitutively expressed by B cells provide the signal for B cells to undergo antibody class switching and produce allergen-specific IgE [9, 10], a process referred to as allergic sensitization. Subsequently, IgE molecules now present in high abundance bind with high affinity to Fcε receptors expressed on granulocytes, where they are cross-linked by allergen molecules upon reexposure, leading to mediator release and immediate-type symptoms, such as urticarial, allergic rhinitis, and conjunctivitis. Immediate-type reactivity is followed by late-phase reactions, which typically occur several hours/days after exposure to allergen. During the late-phase reaction, the affected tissue is infiltrated by Th2 cells and other inflammatory cells including eosinophils and neutrophils, which secrete high levels of cytokines, such as IL-4 and IL-5 to promote inflammation [8].

T cells are not only significant for the onset and maintenance of allergic disease but likely also play a key role for the induction of tolerance, which can be achieved by allergen-specific immunotherapy (AIT) and is the only curative treatment for allergic disease to date. Due to the complexity of human T cell responses against allergens, epitopes have only been thoroughly mapped for the most dominant and prevalent allergens. Recently developed laboratory approaches enable us to perform thorough peptide screens, which achieve the identification and immunological characterization of T cell epitopes in known and novel allergenic targets, irrespective of their IgE reactivity [11, 12]. Mapping of T cell epitopes is of high importance: it greatly facilitates the detection, immunological analysis, and phenotypic characterization of allergen-specific T cells in patients suffering from allergic or asthmatic disease as well as providing a tool to monitor the efficacy of allergen-specific immunotherapy (AIT) treatment. While allergen extracts can also be used to stimulate allergen-specific T cell responses, extracts are not standardized resulting in great variability of allergen content between extract batches [13–15], and endotoxin content is often not monitored [16]. Further, processing and presentation of a large number of peptides present in extract limit the abundance of peptides that represent dominant T cell epitopes. It has been reported that allergen-specific T cells in tissues and peripheral blood are of very low frequency [17, 18], ranging from approximately 10−5 to 10−3 CD4+ T cells, outside or within the pollen season, respectively. The rarity of these cells poses a great challenge for immune mechanistic studies designed to probe how allergic pathology or tolerance induction during AIT administration is orchestrated. The identification of dominant T cell epitopes can therefore be of great importance not only to understand the molecular entities targeted by allergen-specific T cells but also to use them as a tool to detect, isolate, and characterize allergen-specific T cells.

The frequency of patients harboring IgE responses against a specific allergen is most often known and used for classification of the allergen as a minor or major allergen in a respective population [19, 20]. In contrast, T cell epitope data is only available for a small subset of allergens listed by the International Union of Immunological Society (IUIS) database [12]. The relative lack of data on allergen T cell epitopes is likely due to the highly complex nature of T helper cell responses in allergic disease, which makes it a difficult system for immunological studies. Moreover, allergen-specific T cells occur at a very low frequency in the peripheral blood [18], making them hard to detect and isolate. Nevertheless, immunological studies on the allergic T cell response in humans have become of growing importance over the last years. Accordingly, new technologies and concepts have been developed to overcome the challenges of studying allergen-specific T cell responses, map single epitopes, and phenotypically characterize peptide-specific T cells to gain more insights into how T cells contribute to the pathology of allergy and asthma.
