Allergic diseases are a rising global health concern, affecting nearly 30% of the world's population, with variations across age groups and geographic regions. The complexity of allergenic sources—ranging from pollens and foods to venoms and molds—combined with individual immune variability, has long hindered precise diagnosis and targeted management. However, the advent of molecular allergology has introduced an unprecedented shift toward precision diagnostics by identifying specific allergenic proteins, or molecular allergens (MAs), responsible for IgE-mediated hypersensitivity reactions.
Fig.1 Concepts of allergenic source, allergen extract, molecular allergen, and IgE epitope. (Giusti D., et al., 2024)
Immunopathogenesis of IgE-Mediated Allergy
IgE-mediated allergic diseases progress through two phases: sensitization and effector response.
During the sensitization phase, allergens cross epithelial barriers, are taken up by dendritic cells, and are presented to naïve T cells, which differentiate into Th2 cells under the influence of interleukin-4 (IL-4). These cells stimulate B-cell isotype switching to produce allergen-specific IgE antibodies, which bind FcεRI receptors on mast cells and basophils, priming them for rapid activation.
Upon re-exposure to the allergen, cross-linking of IgE-FcεRI complexes on mast cells triggers immediate degranulation, releasing histamine, proteases, and other proinflammatory mediators. A late-phase reaction follows, characterized by leukocyte recruitment and cytokine-driven inflammation, contributing to chronic allergic disease progression.
From Extracts to Molecules: Paradigm Shift in Diagnostics
Traditional allergy testing has relied on allergenic extracts, complex mixtures derived from pollen, foods, or venom, with significant variability in protein content and diagnostic reliability. Many proteins in these extracts are denatured, lost, or underrepresented, such as oleosins in peanuts or peamaclein Pru p 7 in peach, leading to false negatives or inaccurate results.
Molecular allergology overcomes these limitations by using purified, recombinant, or synthetic MAs, allowing:
- Higher diagnostic specificity
- Identification of cross-reactivity vs. genuine sensitization
- Improved risk stratification
- Selection of tailored immunotherapy plans
MA testing is exclusively available for in vitro diagnostics (IVD), using fluorescence, chemiluminescence, or microarray platforms.
Molecular Allergen Families and Their Clinical Impact
Molecular allergens are classified into over 180 biochemical families, each associated with distinct sensitization profiles, exposure routes, and clinical outcomes.
Table 1. Selected Allergen Families and Clinical Correlates.
| Family |
Source |
Exposure |
Clinical Relevance |
| PR-10 |
Birch, apple, hazelnut |
Food, Airborne |
Oral allergy syndrome |
| nsLTP |
Peach, olive, peanut |
Food |
Systemic reactions, severe anaphylaxis |
| Profilins |
Pollens, fruits |
Food, Airborne |
Panallergens, cross-reactivity |
| Lipocalins |
Cat, dog, milk |
Airborne, Food |
Asthma, respiratory allergy |
| Tropomyosins |
Mites, shrimp, cockroaches |
Food, Airborne |
Cross-reactive food and respiratory allergies |
| Storage Proteins |
Peanut, sesame, tree nuts |
Food |
Severe reactions, diagnostic markers |
Marker Allergens: Identifying the True Culprit
Marker allergens are uniquely expressed proteins within a specific allergenic source and serve as diagnostic signatures of genuine sensitization.
For instance:
- Fel d 1 identifies cat dander sensitization.
- Bos d 5 indicates cow's milk allergy.
- Api m 1 confirms honeybee venom sensitization.
- Ole e 1 targets olive tree pollen allergy.
These MAs are critical for initiating allergen-specific immunotherapy (AIT), dietary avoidance plans, and personalized risk mitigation.
Cross-Reactive Allergens and Panallergens
Cross-reactivity arises when IgE antibodies target structurally similar epitopes across unrelated allergenic sources. Panallergen families such as profilins, tropomyosins, and polcalcins are frequently implicated.
Clinical scenarios include:
- Pollen-food syndrome: IgE to Bet v 1 (birch) cross-reacts with Mal d 1 (apple), Pru p 1 (peach), and Cor a 1 (hazelnut).
- Animal dander: Cross-reactivity between Fel d 2 (cat serum albumin) and Can f 3 (dog) or Bos d 6 (cow) can lead to multiple sensitizations.
- Seafood allergy: Shared tropomyosins in shrimp, mites, and cockroach trigger both food and respiratory symptoms.
Understanding cross-reactivity is vital to avoid overdiagnosis and to refine AIT eligibility.
Clinical Utility: From Risk Prediction to Therapy Guidance
Disease Severity and Persistence
Certain MAs predict severe allergic reactions and persistent disease:
- Ara h 2 and Ara h 6 (peanut 2S-albumins): Linked to systemic responses.
- Cor a 9 (hazelnut 11S-globulin): High levels indicate persistence.
- Bos d 8 (milk caseins): Predict non-resolving cow's milk allergy.
- Gal d 1 (ovomucoid): Associated with lasting egg allergy.
Phenotyping Allergic Asthma
Sensitization to Fel d 1, Fel d 4, and Can f 1/2 correlates with asthma phenotypes, especially those with type 2 inflammation. Elevated IgE to lipocalins further predicts refractory or persistent symptoms.
Prognostic Markers
Early detection of Phl p 5 (grass pollen) or Der p 1 (dust mites) sensitization in preschoolers predicts the onset of adolescent asthma, guiding preventive strategies and immunotherapy timing.
Analytical Techniques: Singleplex vs Multiplex Platforms
Singleplex Testing
- Measures IgE to one allergen per assay
- Fully quantitative (e.g., ImmunoCAP, NOVEOS)
- Result unit: kUA/L
- Detection threshold: typically, 0.10 kUA/L
Multiplex Testing
- Measures IgE to >100 allergens from a single 20 µL serum sample
- Includes platforms like ISAC (Immuno Solid-phase Allergen Chip)
- Semi-quantitative (arbitrary units)
- Ideal for polysensitized or complex cases
Image analysis on page 9 illustrates these two workflows, highlighting the advantage of simultaneous profiling in multiplex formats and quantitative rigor in singleplex systems.
Diagnostic Strategy: Integration of MA in Clinical Workflow
Stepwise Diagnostic Approach
- Clinical History: Confirm immediate hypersensitivity to a suspected allergen (symptom onset <4 hours).
- Initial Screening: Skin prick tests or IgE to allergenic extracts.
- MA-Based Profiling:
-Confirm genuine sensitization
-Differentiate cross-reactivity
-Assess risk of severity or persistence
- Personalized Management: AIT, dietary exclusion, or emergency planning
Real-World Case Examples
- Case 1: A woman with fruit allergy and pollen sensitization tested positive for Pru p 3, not Bet v 1, confirming nsLTP-driven systemic allergy, guiding dietary restrictions and AIT targeting ash tree pollen.
- Case 2: A man stung by insects with unclear reaction history showed IgE to Api m 1 but not Ves v 5, confirming honeybee allergy and ruling out wasp venom cross-reactivity, thus initiating honeybee-specific AIT.
- Case 3: A boy with peanut and hazelnut reactions had elevated Ara h 2 and Cor a 9, revealing dual genuine sensitizations, prompting an oral food challenge and AIT planning.
Limitations and Future Perspectives
Despite its strengths, molecular allergology still faces hurdles:
- Underrepresentation: Many fungi, meats, and tropical allergens lack characterized MAs.
- Cost and access: High-tech assays are not universally available or reimbursed.
- Clinical literacy gap: Many clinicians lack training in interpreting MA profiles.
Cross-disciplinary collaboration, involving immunologists, laboratory experts, and allergists, is critical to fully integrate molecular testing into routine care. Europe is spearheading interdisciplinary allergy units to close this gap and expand the use of MA-based diagnostics.
Conclusion: Toward True Precision in Allergy Medicine
Molecular allergology has transformed allergy diagnostics from a generalized, symptom-based discipline into a molecularly guided, stratified, and personalized specialty. By decoding the IgE response at the molecular level, clinicians can distinguish between genuine and cross-reactive sensitizations, stratify patients by risk, and guide tailored therapy, including AIT.
As the field matures and becomes more accessible, molecular allergen testing is poised to become the gold standard in allergy diagnostics, shaping the next era of precision immunology and personalized medicine.
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Reference
- Giusti, Delphine, et al. "Molecular allergology: a clinical laboratory tool for precision diagnosis, stratification and follow-up of allergic patients." Clinical Chemistry and Laboratory Medicine (CCLM) 62.12 (2024): 2339-2355.
This article is for research use only. Do not use in any diagnostic or therapeutic application.
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