**4. Limitations of sIgE determinations: allergen cross-reactivity**

Something to consider when handling sIgE quantification is the cross-reactivity of certain allergens. Some allergens share amino acidic sequences that can be recognized by the same IgE antibody, and this phenomenon is called "cross-reactivity." Cross-reactivity occurs mainly in aeroallergens and food allergens. It is considered to have a high chance of cross-reactivity when two allergens share 35% homology in an 80-amino acid sequence or full identity in a 6–8 amino acid peptide; also, there may be a cross-reaction when the IgE is specific for carbohydrate moiety in the allergen. Hence, laboratory blood tests may detect antibodies to allergens even if the patient has never been exposed to them [37].

Importantly cross-reactivity can occur between allergens from the same family like in nut allergens or in different species of house dust mite; but also, cross-reactivity could be present in diverse phylogenetic sources like house dust mite and shrimp, birch and apple, or fish and chicken meat (**Table 3**) [38–40]. As we read in the previous section, technology innovation through recombinant allergens and full automatization notably reduce cross-reactivity risk when performing sIgE determinations.


**Table 3.** Allergen cross-reactivity and antigens involved.
