Friday, December 20, 2013

Identifying the Unknown Contributors to Shellfish Allergies

According to a 2004 study in the Journal of Allergy and Clinical Immunology, seafood allergies are reportedly present in 2.3% of the general population, or approximately 6.6 million Americans (Sicherer, Munoz-Furlong, and Sampson, 2004).  This represents a serious health concern for the U.S.  With seafood – notably shellfish – consumption having risen in popularity and frequency globally, it has become pertinent that shellfish allergies become better characterized (NOAA, 2013).
     The manifestation of shellfish allergies can be highly variable with symptoms ranging from hives, tingling or swelling of the lips, tongue or throat, chest tightness, shortness of breath or difficulty breathing, nausea and vomiting, to full-blown anaphylaxis (Cleveland Clinic,2012). The allergens associated with shellfish allergies are not well characterized and thus management of such an allergy is often simply limited to avoidance or dietary elimination of shellfish.   Additionally, treatment is restricted to emergency care following exposure (Lieberman et al.,2010). So far, it is known that there are heat stable antigens within shellfish that bind to human IgE, an immunoglobulin or antibody that likely originally evolved as a defense against internal parasites such as helminthes and now significantly contributes to immune-mediated hypersensitivity reactions. Once bound to an allergen, IgE initiates intracellular signaling, leading to the degranulation of immune cells. Degranulation is the release of antimicrobial cytotoxic molecules and mediators of inflammation, which in this case eventually leads to the previously described symptoms. One major type of shrimp allergen that has been identified is tropomyosin, a protein associated with the thin filaments in muscle cells and microfilaments in non-muscle cells. However, there are many other IgE reactive shellfish proteins that have yet to be identified
     A recent study published in PLOS One sought to identify and study different IgE-reactive components of commonly eaten shellfish.  The investigators primarily sought to compare the IgE reactivity of raw and heated proteins of the blue swimmer crab and the black tiger prawn. By treating whole blood and blood sera of individuals with and without shellfish allergies with raw and cooked shellfish extracts, investigators were able to quantifiably measure the degree to which IgE reactivity occurred in response to treatment and to identify unique IgE reactive proteins.
     What was found was that cooked shellfish extracts were much more IgE reactive than raw extracts.  It is possible that this higher level of reactivity is a result of people being more likely to have previously consumed cooked crab or prawn and that their immune systems were more compromised when exposed to these heat modified proteins.  Additionally, it was found that another type of tropomyosin, the crab protein Por p 1, triggers IgE reactions in a similar manner to the black prawn tropomyosin, rPen m 1. A sequence analysis of both proteins showed that they bore a strong genetic homology to one another.  Such homology might allow for cross-reactivity, or the sensitization of an individual to one protein which allows them to have an effector reaction to the other similar protein on a separate occasion.
     What this study shows is that some shellfish allergies can be preserved across shellfish species as a result of the presence of homologous IgE reactive proteins.  Additionally, heat treating blue swimmer crab proteins can lead to a marked increase in IgE reactivity.  This research provides a basis for future studies, as well as for shellfish allergy management and diagnosis, by better characterizing shellfish allergens and outlining factors that may contribute to more robust allergic reactions.

Helpful Links
To learn more about shellfish allergies, see: http://www.mayoclinic.com/health/shellfish-allergy/DS00987

To learn more about IgE and its role in allergic inflammation, see: http://www.worldallergy.org/professional/allergic_diseases_center/ige/

Primary Source
Abramovitch, J.B., Kamath S., Varese, N., Zubrinich C., Lopata, A.L., O’Hehir, R.E., Rolland, J.M. (2013) IgE reactivity of blueswimmer crab (Portunus pelagicus)Tropomyosin, Por p1, and other allergens; Cross-reactivity with black tigerprawn and effects of heating. PLOS One 8:1-13.

Secondary Sources
Sicherer, S.H., Munoz-Furlong A., Sampson H.A. (2004)Prevalence of seafood allergy in the United States determined by a randomtelephone survey. J Allergy Clin Immunol 114:159-165.   

NOAA. 2013. 2012 U.S. seafood landings remain near high 2011 levels. NOAA. Accessed at http://www.noaanews.noaa.gov/stories2013/20131030_2012usseafoodlandings.html on 20 December 2013.

Cleveland Clinic. (2012) Shellfish allergies. Health Hub from Cleveland Clinic. Accessed at http://my.clevelandclinic.org/disorders/shellfish_allergies/hic_shellfish_allergies.aspx on  20 December 2013.

Lieberman, P., Nicklas R.A., Oppenheimer J., Kemp S.F., Lang D.M.(2010) The diagnosis and management of anaphylaxis practice parameter:2010. Update. J Allergy Clin Immunol 126:477-480.   

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