D3.291 - Molecular Identification of Heat-Stable β-Parvalbumin in megrim (Lepidorhombus boscii) as the Likely Allergen in a Child With Selective Fish Reactivity

Fish allergy is mainly mediated by parvalbumins, although selective sensitization to individual species may occur depending on protein stability and tissue distribution. A major clinical challenge is distinguishing true species-specific allergy from cross-reactive sensitization. Molecular techniques such as SDS-PAGE and IgE–Western blot allow direct identification of IgE-binding proteins and may clarify the molecular basis of selective reactivity.

A 12-year-old boy with atopic dermatitis developed an immediate reaction after ingestion of megrim (Lepidorhombus boscii), consisting of pruritus and lip angioedema despite oral antihistamine treatment. Diagnostic work-up showed discordant results, with positive skin prick tests to megrim and other fish species and negative serum specific IgE, including IgE to recombinant cod parvalbumin. A supervised oral food challenge to megrim was positive. Based on these findings, molecular analysis was performed.

Protein extracts of raw and cooked megrim, tuna, sole (skin and muscle), and hake were prepared. SDS-PAGE and IgE–Western blot were performed under reducing and non-reducing conditions using the patient’s serum.

Skin prick tests (ROXALL) were positive to megrim, tuna and hake, and negative to sole, emperor fish, anchovy, cod and Anisakis. Serum specific IgE (ImmunoCAP, Thermo Fisher) to megrim and other fish species was negative, including IgE to recombinant cod parvalbumin (rGad c 1). The oral food challenge to megrim induced lip edema and erythematous lesions on the face and forearms (oFASS-5 grade 2), resolving after oral cetirizine.

IgE–Western blot showed a consistent IgE-binding band at ~10 kDa in raw and cooked megrim extracts, with increased intensity after cooking, compatible with a heat-stable β-parvalbumin. In tuna extracts, an IgE-binding band at ~40 kDa was detected only in raw samples, compatible with tuna aldolase (Thu a 3); however, the patient tolerated tuna in both raw and cooked forms, indicating sensitization without clinical allergy. In sole skin extracts, high-molecular-weight IgE-reactive bands (~150–100 kDa) were observed despite clinical tolerance, suggesting recognition of non-canonical proteins. No IgE binding was observed in hake extracts.

This case illustrates a selective clinical allergy to megrim mediated by a heat-stable β-parvalbumin despite negative serum specific IgE. Molecular characterization by SDS-PAGE and IgE–Western blot allowed identification of the clinically relevant allergen and highlights the value of molecular approaches in selected cases with discordant conventional diagnostic tests.