Molluscan shellfish allergens

The importance of molluscan shellfish allergy is increasingly recognized. The European Union recently added molluscan shellfish to the list of most commonly allergenic foods in Europe (EESA, 2006). Although not knovm... 

The number of sfudies estimating the comparative prevalence of molluscan shellfish allergy is limited. The frequency of consumption of molluscan shellfish mighf be higher in some of the locales where such studies have been performed. Clearly, more comparative clinical data would be helpful. However, the molluscan shellfish certainly seem to be a comparatively common allergenic food in some locales and among some populations. 

While all humans have IgE antibodies fhaf are involved in defense against parasitic infections, only humans who are predisposed to the development of allergies will produce IgE antibodies upon exposure fo certain protein allergens present in their environment including their diet. Only a few of the many proteins found in foods are capable of sfimulating the production of specific IgE antibodies in susceptible individuals (Taylor, 2002). With molluscan shellfish, only one, or perhaps a few, of the numerous proteins is known to provoke the production of IgE antibodies that specifically recognize one or more species of molluscan shellfish. 

The major allergen of molluscan shellfish is tropomyosin, a muscle protein. The term major allergen is used to define proteins that elicit IgE binding in the sera of half or more of patienfs wifh allergies to the specific source (Metcalfe et ah, 1996). Tropomyosin is a ubiquitous muscle protein in all animals. Tropomyosin is a 34- to 36-kDa protein that is highly water soluble and heat stable as evidenced by the fact that tropomyosin can be isolated from fhe water used to boil shrimp (Daul et ah, 1994). Tropomyosin can actually be found in bofh muscle and many nonmuscle cells in animals. In muscle cells, tropomyosin is associated with the thin filaments in muscle and plays a role in the contractile activity of muscle cells. In nonmuscle cells, tropomyosin is found in microfilaments but its fimction is less well imderstood. Tropomyosins are present in all eukaryotic cells. Different isoforms of tropomyosin are found in different types of muscle cells (skeletal, cardiac, smooth), brain, fibroblasts, and other nonmuscle cells. While these tropomyosins are highly homologous, small differences do exist in their... 

TABLE 4.4 Tropomyosin allergens from molluscan shellfish species... 

Solid evidence exists to indicate that tropomyosins are pan-allergen among invertebrate species (Reese et ah, 1999). However, nontropomyosin allergens also exist in at least some species of molluscan shellfish. The clinical picture of cross-reactivity is more complex than might be anticipated. 

Cross-reactivity between molluscan and crustacean shellfish species also occurs rafher frequently. Since tropomyosin is the major allergen in both molluscan and crustacean shellfish, fhe frequency of cross-reactions is not surprising. Allergy to crustacean shellfish is more frequenfly diagnosed than molluscan shellfish allergy (Hefle et ah, 2007). Many of fhese individuals may be at risk of reactions fo molluscan shellfish also. Appropriately, most individuals with either molluscan or crustacean shellfish allergy are advised to avoid all shellfish. 

The tropomyosins of mite and insect species show some sequence identity (63-65%) with snail tropomyosin and share similar epitopes (EFSA, 2006 Fig. 4.1). Still, tropomyosin appears to play a minor role in the crossreactivity of dust mites and snails (Asturias et ah, 2002 Guilloux et ah, 1998 Van Ree et ah, 1996a). Other non-tropomyosin allergens are likely to be involved including Der p 4 (amylase), Der p 5, Der p 7, and hemocyanin (Martins et ah, 2005 Mistrello et ah, 1992 Van Ree et ah, 1996). While snail is the main molluscan shellfish species involved in cross-reactions with dust mites, some patients allergic to dust mites and snails are also sensitized to mussels (DeMaat-Bleeker et ah, 1995 Van Ree et ah, 1996b). In their study of 70 patients sensitized to molluscan shellfish, Wu and Williams (2004) noted that 90% were also sensitized to dust mites. However, the clinical significance of this sensitization was not documented. 

The only proven therapy for molluscan shellfish allergy is strict dietary avoidance. Problems may arise with avoidance diets when clam is present due to mislabeling or to cross-contact during food processing (Taylor and Hefle, 2005 Taylor et ah, 1986,1999). The food industry typically develops allergen control programs to prevent the occurrence of undeclared allergenic residues in other foods (Taylor et ah, 2006). The industry often uses... 

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