Introduction to Molecular Allergology

Learning objectives

At the end of this chapter, participants will be able to:

  • explain how component-resolved diagnostics can be used alongside traditional sensitisation tests in the clinical setting
  • discuss the strengths and key considerations of using molecular-based platforms for diagnosis.

Defining allergens

  • An allergen is a substance (usually a protein) that can bind to immunoglobulin E (IgE) antibodies.1

Types of allergen

  • Allergens can be further classified into:1

 

 

Protein families and cross-reactivities

  • Proteins can be classified into protein families based on the similarity of their amino acid sequences and three-dimensional structures.3
  • Despite the majority of allergens being proteins, not all proteins are allergenic. Only a few protein families contain allergens, such as polcalcin, profilin, serum albumin, lipocalin, parvalbumin, pathogenesis-related and the non-specific lipid transfer protein families.1
  • Proteins within the same family can have similar IgE and T-cell recognition sites (epitopes) despite being present in different allergen sources.2 This leads to the same IgE antibodies binding to several proteins and inducing allergic responses to allergen sources to which an individual may have never been exposed;2 these immunological responses are defined as cross-reactions.2,3
  • Cross-reactivities generally require a sequence identity of >50% between proteins and can provide valuable information on potential sensitisation and clinical reactions to several different sources.3
  • Additionally, patients can be sensitised to multiple allergens (where sensitisation is not due to cross-reactivity) that induce several IgE-mediated immunological responses, termed co-sensitisation.4

Antibodies can cross-react with unrelated antigens in proteins if they share identical or similar epitopes.

Allergen components

  • Advances in molecular biology and protein biochemistry have led to the availability of purified or recombinant single allergen components (referred to as “single allergens” or “components”) to support allergy diagnosis.1,3
  • Use of such components in allergy diagnosis can lead to:1,3
    1. improved standardisation
    2. greater analytical sensitivity of in vitro tests
    3. improved analytical specificity and selectivity for associating allergens with risk
    4. identification of species-specific allergens for the detection of true sensitisation
    5. identification of allergens involved in cross-reactions.

Molecular allergology – component-resolved diagnostics (CRD)

  • Mass production and availability of more than 130 allergen components2 have paved the way for a new in vitro IgE diagnostic approach called component-resolved diagnostics (CRD).6
  • CRD involves the measurement of specific IgE levels to individual allergen component(s). There are two separate formats in which CRD can be used:1

Diagnostic algorithms including CRD1

Advantages of using CRD in routine diagnostics

Use of single allergens in diagnostics can improve clinical interpretation. For example, CRD may be used to:

  • provide information on the patient’s specific allergen sensitisation profile, including primary (genuine) sensitisations and cross-reactivities6–8
  • aid decisions on providing patients with the correct immunotherapy – especially in cases of polysensitisation8,9
  • potentially avoid the immediate need for provocation testing.10

Key considerations when using CRD

CRD offers additional benefits to the use of allergen extract-based diagnostics. However, several recommendations should be considered when selecting CRD and interpreting the data produced:

  • The patient’s clinical history and symptoms should be used to select the correct allergen components for assays.11
  • All results should be interpreted using the patient’s medical history and symptoms.11
  • A positive result for IgE sensitisation against an allergen component does not causally imply that this allergen is clinically relevant.12 The patient’s clinical history and symptoms must also support such interpretation.11
  • Results cannot be extrapolated from one patient to the rest of the population – this a personalised diagnostic tool.13
  • If specific IgE is not detected in the serum, this rules out sensitisation.3
    1. However, the following factors must be met to rule out sensitisation: serum total IgE levels (>20 kU/I), tested suitability of the allergen component in diagnosis and sensitivity of the IgE testing method.
  • Multiplex assays can be less sensitive than singleplex assays at low serum IgE levels.8

References

  1. Matricardi PM et al. (Editors). Molecular Allergology User’s Guide. 2016. Zurich: European Academy of Allergy and Clinical Immunology.
  2. Canonica GW et al. World Allergy Organization Journal. 2013;6(17):1–17.
  3. Kleine-Tebbe J, Jakob T (Editors). Molecular Allergy Diagnostics. Springer Nature. 2017.
  4. Migueres M et al. Clinical and Translational Allergy. 2014;4(16).
  5. https://microbionotes.com/antigen-antibody-reaction/ (Accessed May 2019)
  6. Valenta R et al. Clinical and Experimental Allergy. 1999;29(7):896–904.
  7. Valenta R et al. Immunology and Cell Biology. 1996;74(2):187–94.
  8. Jakob T et al. Allergo J Int. 2015;24:320–32.
  9. Stringari G et al. J Allergy Clin Immunol. 2014;134(1):75–81.e2.
  10. Dang TD et al. J Allergy Clin Immunol. 2012;129(4):1056–63.
  11. Portnoy JM. Missouri Medicine. 2011;108(5):339–43.
  12. Alessandri C et al. Clin Transl Allergy. 2017;7(21).
  13. Dodig S, Čepelak I. Biochemia Medica. 2018;28(2): z1–z9.

 

 
PCM Scientific, 140 London Wall, London EC2Y 5DN