Search for a command to run...
IgE-mediated allergic reactions are triggered by a precisely defined molecular process, the allergen-induced cross-linking of IgE antibodies bound to the high-affinity receptor FcεRI on the surface of mast cells and basophils. While clinically informative parameters such as serum IgE levels and allergen exposure thresholds are valuable, mounting evidence indicates that the qualitative characteristics of IgE-allergen interactions, including epitope specificity, affinity, spatial organization, and cross-reactivity, are critical factors in determining whether an allergen encounter leads to strong effector-cell activation and clinical symptoms [1]. Among food allergies, peanut allergy is characterized by low reaction thresholds and a high risk of anaphylaxis, driving extensive research into the molecular determinants underlying the potency of allergens. Within the complex repertoire of peanut allergen, the 2S albumins Ara h 2 and Ara h 6 and the cupin superfamily members Ara h 1 (vicilin) and Ara h 3 (legumin) are classified as major allergens based on their IgE-binding frequency. Although co-sensitization to all four allergens is associated with more severe clinical manifestations, Ara h 2-specific IgE appears to dominate the allergic reactions [1]. This suggests Ara h 2 possesses intrinsic molecular properties conferring exceptional potency. Structurally, Ara h 2 contains a disulfide-stabilized core typical of 2S albumins but is uniquely characterized by a large, flexible surface loop containing repeated DPYSPS motifs in which the second proline residues are hydroxylated [2] (three in Ara h 2.0201; two in Ara h 2.0101). Population studies have identified these motifs as “public” epitopes that serve as prognostic markers for allergy severity [3] and as predictors of immunotherapy outcomes [4]. Another characteristic of Ara h 2 is its unexplained cross-reactivity with the structurally unrelated Ara h 1 and Ara h 3 [5, 6]. In this issue of Allergy, Parkkinen and colleagues provide a comprehensive structural and mechanistic explanation for the exceptional potency of Ara h 2 and the basis for this unusual cross-reactivity [7]. By solving the crystal structure of recombinant Ara h 2.0201 in complex with a patient-derived IgE Fab fragment (clone PA12P3D08), the study presents the first atomic-level structure of a food allergen that binds three IgE antibodies simultaneously. The three identical IgE Fab fragments bind to the three closely spaced, repeated DPYSPS motifs forming a quaternary immunocomplex. The measured inter-epitope distances (approximately 1.5–2.3 nm) are significantly shorter than those inferred from classical synthetic hapten systems [8]. This research demonstrated that trivalent haptens are at least 1000-fold more effective than bivalent ones at triggering mast cell degranulation, underscoring the critical importance of epitope valency. Despite being short linear sequences, the proline-induced conformational restriction and motif repetition generate strong avidity and an exceptionally high apparent affinity for IgE in the sub-picomolar range [5], which is among the highest reported for antibody–antigen interactions. Furthermore, the compactness of the complex may prolong its lifespan and facilitate enhanced antigen uptake and presentation by B cells, potentially amplifying Ara h 2-specific IgE responses. A key aspect of the Parkkinen study is its focus on the monoclonal IgE antibody PA12P3D08, which belongs to a convergent (“public”) clonotype repeatedly identified in several peanut-allergic cohorts [5, 9]. In murine models, sensitization with this antibody alone was sufficient to induce anaphylaxis upon peanut challenge [8]. Functional assays confirm that binding of a single Ara h 2 molecule by a single IgE from this clonal family can trigger effector cell activation without the need for additional antibody specificities. Furthermore, antibodies from this clonal family exhibit cross-reactivity with the Ara h 2 unrelated major peanut allergens Ara h 1 and Ara h 3 [5, 6]. A second important contribution of this study is the first mechanistic explanation for IgE cross-reactivity between non-homologous peanut allergens caused by a post-translational modification (PTM). Native mass spectrometry and peptide-binding studies demonstrated that hydroxylation of the second proline within the DPYSPS motif significantly increases Fab affinity by approximately threefold [7]. This finding explains why bacterially expressed recombinant Ara h 2 and its synthetic peptides, which lack proline hydroxylation, often show reduced IgE binding [6, 10]. Crucially, the same Ara h 2-specific IgE antibody binds to short PTM-containing motifs within Ara h 1 and Ara h 3, despite minimal primary sequence identity (Figure 1). The shared structural motif consists of a short tripeptide flanked by an aromatic residue and a hydroxyproline. These motifs are mostly present in surface-exposed loops of these three allergens [2]. Binding affinities, strongest for Ara h 2 and Ara h 3-peptides, and weakest for Ara h 1, mirror the relative potencies of these allergens in functional assays [6]. The long, flexible complementarity-determining region H3 loop of the antibody could enable accommodation of these chemically similar but sequentially distinct epitopes [7]. This also suggests a mechanism for heterotypic IgE cross-linking: one Fab arm of a bivalent IgE can bind with high affinity to an Ara h 2 epitope, while the other Fab arm engages with lower affinity to a similar PTM-bearing motif on the more abundant Ara h 1 or Ara h 3. Such bivalent interactions substantially increase the overall avidity and efficiency of FcεRI cross-linking on effector cells, thereby amplifying allergic reactions. This provides another compelling explanation for the allergenic potency of peanut. Furthermore, these antibodies also recognize legumin allergens from tree nuts (Brazil nut, walnut, almond) [6], likely accounting for frequent, though often clinically less relevant, co-sensitizations observed in peanut-allergic patients. These findings may also be applicable to other food allergies. For instance, the immunodominance of the 2S albumin Ana o 3 in cashew allergy and its extensive cross-reactivity with cashew vicilin and legumin [11] could reflect a similar PTM-dependent mechanism. In summary, Parkkinen and colleagues provide important new insights that bridge the gap between allergen structure and clinical phenotype in peanut allergy. By elucidating the interactions between trivalent IgE-binding and specific post-translational modifications, this study explains both the extreme potency of Ara h 2 and the basis for cross-reactivity between the major peanut allergens. Importantly, it underscores that immunological recognition depends not only on amino acid sequence but also on molecular authenticity, including correct folding and PTMs. This is a fundamental insight that must guide future allergen research, development of diagnostic tools, and the design of novel therapeutics. Both authors contributed to the writing and critical review of the manuscript. Author Heimo Breiteneder acknowledges the support of Danube-ARC project P04 funded by the Federal State of Lower Austria. The authors have nothing to report. The authors declare no conflicts of interest. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.