Recently I saw a 7 year old male allergic to dust mites and cockroach who also had an 8 mm positive skin test to shrimp though he had some shrimp a month before without any adverse effect. Could the skin test to shrimp indicate antigenic cross reactivity with dust mites and cockroach rather than true allergy? Thank you for your advice and help.


Thank you for your inquiry.

As you can see from the abstracts copied below, shrimp, dust mite, and cockroach all contain tropomyosin with a high degree of structural homology. Therefore these allergens do crossreact. However, it is not the issue of crossreactivity that distinguishes whether or not the skin test is a reflection of clinical reactivity. Tropomyosin is the major allergen in shrimp, and certainly sensitization to tropomyosin can result in clinical reactivity.

However, as you know, there is a high degree of sensitization to foods in general without clinical reactivity to the food. It is not the issue of crossreactivity that determines whether or not clinical reactivity occurs, and unfortunately we do not understand the mechanisms that allow for sensitization without clinical reactivity.

In this case, the allergens clearly crossreact, but that is not the issue that would determine whether or not your patient will react to shrimp.

Thank you again for your inquiry and we hope this response is helpful to you.

Background: Cockroaches produce several proteins that induce IgE antibody responses. Although cockroaches are abundant in warm and humid areas, sensitization to cockroach allergens has not been investigated in Brazil.
Objective: The aims of this study were to investigate the frequency of cockroach allergy among patients with asthma, rhinitis, or both in Brazil and to identify American cockroach allergens.
Methods: Skin tests using cockroach extracts were performed on children and young adults with asthma, rhinitis, or both. A Periplaneta americana complementary (c)DNA library was screened by using IgE antibodies from Brazilian patients allergic to cockroaches. Reactivity of an mAb directed to Dermatophagoides pteronyssinus tropomyosin against cockroach tissue was examined by immunofluorescence.
Results: Cockroach allergy was present in 55% and 79% of the patients, as determined by using skin prick tests alone or combined prick and intradermal tests, respectively. Five cDNA clones reacted with IgE antibody and contained the same sequence. A representative clone (1300 bp), pa 12, coded for a protein that reacted with 50% of the sera from patients allergic to cockroaches on plaque immunoassay and showed a high degree of homology to tropomyosins, particularly those from invertebrates. P americana tropomyosin showed 80%, 81%, and 82% sequence identity to tropomyosins from D pteronyssinus, D farinae, and shrimp, respectively, which have been previously defined as important allergens. An mAb directed against D pteronyssinus tropomyosin, which also recognizes shrimp tropomyosin, showed binding to cockroach striated muscle.
Conclusion: Our results support the recommendation that cockroach extracts should be routinely used for the evaluation of patients with asthma, rhinitis, or both in Brazil. The identification of P americana tropomyosin as an important allergen will make it possible to investigate cross-reactivity among cockroaches, mites, and food derived from invertebrates. (J Allergy Clin Immunol 1999;104:329-37.)

Shrimp, a major seafood allergen, was investigated as a model food allergen. Extracts from both shrimp (Penaeus aztecus) meat and cooking fluid contain a substantial and similar amount of allergenic activity. A 36-kD allergen, demonstrated in both extracts by SDS-PAGE/Western blot analysis, reacted with 28/34 (82%) sera from shrimp-sensitive, skin test and RAST-positive, individuals. This allergen, named Pen a I, was isolated by SDS-PAGE; its amino acid composition was rich in aspartic and glutamic acids. A 21-residue peptide, obtained from endoproteinase Lys-C digested Pen a I by high-performance liquid chromatography, demonstrated significant homology (60-87%) with the muscle protein tropomyosin from various species and origins. The greatest homology (87%) was noted with tropomyosin of the fruit fly (Drosophila melanogaster) reflecting the phylogenic relationship between these two arthropods. These studies demonstrate that tropomyosin is the major shrimp allergen. Although the amino acid sequence of this shrimp muscle protein shares considerable homology with tropomyosins of other species including man, significant differences remain in allergenic activity. (International Archives of Allergy and Immunology 1994, Vol. 105, No. 1)

Phil Lieberman, M.D.

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