Introduction
Fish is one of the “Big 8” food allergens, with IgE-mediated fish
allergy affecting approximately 1% of the world population and a much
higher prevalence in pediatric cohorts at up to 7%1-3. Patients with fish allergy suffer from
symptoms ranging from mild skin rashes to life-threatening
anaphylaxis. Conventionally, fish allergy has been considered an
umbrella term, and patients who are diagnosed to be allergic to one kind
of fish are often advised to avoid all fish. Clinical reactions to
multiple fish species have been mainly attributed to the cross-reactive
major fish allergen, parvalbumin 4 . However,
there are also reports of mono-allergy to single fish while tolerating
others. Our team neatly demonstrated that a significant proportion of
patients are tolerant to salmon despite experiencing severe allergic
reactions to grass carp at double-blind, placebo-controlled food
challenges (DBPCFC) 5 . Selection of appropriate
fish for IgE testing and dietary reintroduction remains a major clinical
gap due to the wide range of fish species from evolutionary distant
classes consumed worldwide.
Parvalbumin is a 12-kDa sarcoplasmic calcium-binding protein for muscle
contraction. It accounts for up to 95% of sensitization in patients
with IgE-mediated fish allergy 6,7 . The
capability of recombinant carp parvalbumin to induce the release of
histamine from basophils of patients allergic to fish has long been
reported 8 . Its high allergenicity (the ability
to stimulate allergic reactions) can be attributed to its supreme
stability even after cooking at high temperatures9 . The activity of parvalbumin was also
retained after treatment of denaturing agents, meaning that its
allergenicity depends primarily on its amino acid sequence instead of
higher-level structures. Parvalbumin has been shown to be involved in
fast muscle relaxation, in which parvalbumin level and relaxation speed
of muscles are closely and positively correlated10 . It can mainly be found in fast-twitching
white muscles, while endurable dark muscles in fish contain less
parvalbumin. Therefore, migratory fish, such as tuna, have comparatively
more dark muscle and hence less parvalbumin than sedentary fish, which
have more white muscle instead 11 .
Differences in the protein levels of parvalbumin correlated with the
difference in IgE reactivity to fish12 . Clinical studies have indicated that sera
of fish-allergic patients were more reactive to white muscle extract
than to dark muscle extract, in which parvalbumin level was four to
eight times lower 13 . However, a comprehensive
large-scale study detailing the clinical reactions to multiple fish
species, analysis of serological IgE reactivity and their correlation to
the parvalbumin level of the corresponding fish, and the identification
of fish-specific IgE-binding epitopes for clinical cross-reactivity
prediction is lacking. Herein, we report the clinical results of 200
fish-allergic subjects (specific IgE distribution, self-reported
reactions to fishes, and DBPCFC data) and molecular analysis
(parvalbumin levels determined by both protein and transcriptomic
approaches and IgE reactivity of recombinant parvalbumins) for the
construction of fish allergenicity ladder, as well as IgE-binding
epitopes for clinical cross-reactivity prediction. Such ladder and the
epitope results can stratify fish-allergic patients and the
allergenicity difference of fishes to direct the selection of fishes for
improving diagnosis and the gradual reintroduction of fishes in patients
with IgE-mediated fish allergy.