Parvalbumins are the most important fish allergens, mediate cross-reactivity between fish species

Parvalbumin is a calcium binding protein with low molecular weight structurally related to calmodulin and troponin C. Parvalbumin is localised in fast-contracting muscles, brain and some endocrine tissues.

Parvalbumins are the most important fish allergens. Polysensitization to various fish species is common and linked to the cross-reactivity of their parvalbumins.

Dark-muscled fish such as tuna may be less allergenic.

Total protein extracts and purified parvalbumins from cod, whiff, and swordfish were tested for IgE-binding properties with 16 fish-allergic patients' sera from Spain.

Parvalbumins levels from cod were 20 times higher than from swordfish (whiff 30 times higher).

Parvalbumins were recognized by 94% of the patients in extracts of cod and whiff, but only by 60% in swordfish extracts.

The parvalbumins of cod, whiff and swordfish are highly cross-reactive due to high amino acid sequence.

The low allergenicity of swordfish is due to the low expression levels of its parvalbumin.

Fish-allergic patients should avoid all fish species until a species can be proven safe to eat by provocative challenge (Annals of Allergy and Imm, 1999).

References:

Expression levels of parvalbumins determine allergenicity of fish species. U. Griesmeier, S. Vázquez-Cortés, M. Bublin, C. Radauer, Y. Ma, P. Briza, M. Fernández-Rivas, H. Breiteneder. Allergy, 2009.
Epitope Mapping of Atlantic Salmon Major Allergen by Peptide Microarray Immunoassay http://goo.gl/OGCnn

Cross Reactions Among Foods (PDF).
Image source: Gadus morhua, Atlantic cod. Wikipedia, public domain.