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Wednesday 24 April 2019
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Expert analysis: Allergy sensitisation - the diagnostic approach in 2019

Allergies are the most common immune system disorders. Their prevalence is continuously increasing, especially in Western countries, where they affect more than 25% of the population.1

Allergies are abnormal responses to exogenous molecules (the allergens), normally not responsible for pathological events in healthy subjects. Allergens can be classified depending on the sources (that is, inhalants, food, latex, Hymenoptera venoms, etc)2 and they can be subdivided into different families, each containing different molecules with a structural analogy.2

Allergies lead to different manifestations in target organs, including asthma, rhinitis, conjunctivitis, urticaria, anaphylaxis, etc. The severity of allergic events ranges from very mild to fatal.1

Sensitisation does not mean allergy
Allergy is a multistep process: symptoms do not arise during the first encounter with an allergen. In a predisposed subject (atopic), the allergen triggers an abnormal immune reaction leading to production and secretion of immunoglobulin E (IgE), a particular class of antibodies.

IgE bind to their specific receptors on the surface of tissue-resident cells, the mastocytes; this is called sensitisation phase. Mastocytes play a crucial role: indeed, they are responsible for the production of inflammatory mediators, such as histamine, prostaglandins, and leukotrienes, the principal players in allergic reactions.3

The presence of specific IgE to one or more allergen families is not always related to the allergic disease. But a sensitised patient is also an allergic patient if symptoms can be strictly related to the sensitisation observed. Thus, allergic symptoms arise when allergens bind IgE on the surface of the mastocytes, triggering the secretion of the inflammatory mediators mentioned above. Effects are extremely rapid and include vasodilation, increasing in capillary permeability (tissue oedema), itching, bronchoconstriction, and increase in mucus secretion, etc. If the reaction is systemic, the patient could undergo anaphylactic shock, a life-threatening condition.3

The diagnostic process
Diagnosis can be carried out at four levels.

Level 1
The first level is a medical examination followed by skin prick tests (SPTs), considered the standard diagnostic procedure.4,5 SPTs are performed on the forearm: and involves subcutaneous injection of a series of allergen extracts via a needle or lancet, alongside a negative (saline) and a positive (histamine) control. A positive result is defined by the appearance of a wheal of at least 3mm in diameter.4 SPTs are relatively inexpensive and specific, but there are some limitations: each extract is a heterogeneous mixture of both allergenic and non-allergenic molecules, the patient must suspend any anti-histaminic treatment before the test, skin must be intact and, as it is an in vivo test, it may present some risks.4-6

Level 2
The second level is represented by specific IgE (sIgE) assays for allergen sources. The tests are performed in vitroon patient serum, to find antibodies specific to a given allergen. It is a single-plexed approach: a few allergens are tested, thus only what is investigated is addressed.6–8 When the raw allergen extract is substituted by an allergen molecule (component), sIgE tests are considered third-level diagnostics.

Level 3
Third level diagnostics are also represented by allergen arrays (AAs), which allow the determination of an extended IgE profile.2,9 Good examples of AAs are the ImmunoCAP ISAC (Thermo Fisher) and the ALEX (ALLergy EXplorer, MADx). ISAC is the first AA, and its revolutionary impact on allergy diagnostics is well documented.2,6,9 ALEX has recently been introduced and is an innovative multiplexed AA, containing 282 allergens,8 of which 156 are raw extracts, and 126 are components. Thus, ALEX combines second and third diagnostic levels. Of note, AA allows the identification of sensitisation to genuine and/or cross-reactive components. Cross-reactivity occurs when IgE specific to an allergen shows reactivity even against another that shares a similar primary sequence. AAs are particularly useful in cases of poly-sensitisation, which, to date, is common in the majority of allergic patients. Also, the discrimination between genuine and cross-reactive components is essential to programme personalised allergy immunotherapies.2,10,11 The counterpart of these innovative diagnostic systems is the enormous amount of complex data. Artificial intelligence and expert systems (such as Allergenius) are proving to be useful.12

Level 4
The fourth level is based on functional tests such as the basophil activation tests,13 assays usually confined to specialist reference centres.

Conclusions
In conclusion, diagnostic procedures in allergy have been greatly modified (and improved) by molecular allergy tools that not only identify the molecular allergen that causes sensitisation but that also provide personalised therapeutic solutions, to achieve the best possible clinical outcome.

References

  1. Pawankar R et al. Chapter 2. The burden of allergic diseases. In Pawankar CG et al (eds) World Allergy Organization (WAO): White Book on Allergy: Update 2013. World Allergy Organization 2016:27–33.
  2. Canonica GW et al. A WAO -ARIA - GA(2)LEN consensus document on molecular-based allergy diagnostics. World Allergy Organ J 2013;6(1):17.
  3. Metcalfe DD et al., Biomarkers of the involvement of mast cells, basophils and eosinophils in asthma and allergic diseases. World Allergy Organ J 2016;9:7.
  4. Kowalski ML et al., Risk and safety requirements for diagnostic and therapeutic procedures in allergology: World Allergy Organization Statement. World Allergy Organ J 2016;9(1):33.
  5. Matricardi PM et al. EAACI Molecular Allergology User's Guide. Pediatr Allergy Immunol 2016;27 Suppl 23:1–250.
  6. Jensen-Jarolim E et al. Debates in allergy medicine: Molecular allergy diagnosis with ISAC will replace screenings by skin prick test in the future. World Allergy Organ J 2017;10(1):33.
  7. Heffler E et al. Microarray Immunodiagnostics for Aeroallergens. Curr Allergy Asthma Rep 2019;19(1):10.
  8. Heffler E et al. Extended IgE profile based on an allergen macroarray: a novel tool for precision medicine in allergy diagnosis. World Allergy Organ J 2018; 11(1):7.
  9. Melioli G et al. The ImmunoCAP ISAC molecular allergology approach in adult multi-sensitized Italian patients with respiratory symptoms. Clin Biochem 2011;44(12):1005–11.
  10. Passalacqua G et al. The additional values of microarray allergen assay in the management of polysensitized patients with respiratory allergy. Allergy 2013;68(8):1029–33.
  11. Riccio AM et al. Molecular diagnosis and precision medicine in allergy management. Clin Chem Lab Med 2016;54(11):1705–14.
  12. Melioli G et al. Allergenius, an expert system for the interpretation of allergen microarray results. World Allergy Organ J 2014;7(1):15.
  13. Hoffmann HJ et al. The clinical utility of basophil activation testing in diagnosis and monitoring of allergic disease. Allergy 2015;70(11):1393–405.