gastronomy is not a world only of pleasurable aromas and tastes. More than one-quarter of the population in seven countries of the European Union claims to suffer from food allergies or intolerances. Although clinical tests indicate that the actual incidence of such allergies is much lower than commonly believed (about 3.5% of the population), these reactions pose a major public health problem, all the more because the seriousness of the attacks reported is on the rise. In the last ten years the number of cases of ana-phylactic shock caused by a food allergy has quintupled, and many of these attacks are fatal, notably ones triggered by the ingestion of products derived from peanuts.
Jean-Michel Wal and his colleagues at the Institut National de la Recherche Agronomique-Commissariat de l'Énergie Atomique (inra-cea) Laboratoire d'Immuno-Allergie Alimentaire in Saclay have shown how a protein found in cow's milk triggers allergies despite its resemblance to a human protein that is readily tolerated. They are also conducting animal tests of a method of gene immunization for preventing allergies.
The reasons for the recent growth of food allergies are not completely understood. The increase in the number of offending foods seems to be linked to the development of respiratory allergies to plant pollens; the antibodies produced by the human organism against a pollen antigen sometimes react also against molecules from a very different source. These cross-reactions are a product of shared epitopes, molecular fragments that are the target of the immune system. Thus the multiplication of allergies to exotic fruits (avocados, kiwi fruits, bananas, and so on) has been found to be associated with the development of allergies to latex, themselves on the rise as well.
To these traditional foods have been added new ones derived from genetically modified organisms, which contain proteins normally absent in traditional foods. The growing commercial availability of such products has aroused concerns that they may be allergenic, but because few cases of illness have been reported until now it is not clear how to assess the risk posed by the introduction of new foods. Several approaches are being investigated: tests involving serums from allergic patients (whose antibodies bind to allergenic molecules), cutaneous tests (in which molecules thought to be allergenic are deposited on the skin, and signs of adverse reaction are monitored), animal studies (in which candidate molecules are administered to sensitized animals), and theoretical models of the incidence of allergic reactions based on the protein sequence in amino acids.
Developing such models will take time, but the preliminary step of compiling sets of well-characterized data for proteins whose allergenic character is known is now being carried out. Wal and his colleagues have already used these databases to compare the immune reactivity of human and bovine beta-casein.
Aller^es to Milk
Caseins constitute 80% of the proteins found in milk. There are four types: alpha S1, alpha S2, beta, and kappa. The beta-casein in cow's milk is an important allergen: 92% of the serums obtained from people allergic to caseins contain a type of antibody responsible for allergic reactions, known as immu-noglobulin E, that is specifically directed against beta-casein.
In 1997 the immunologists at Saclay demonstrated the presence of immunoglobulin E molecules specific to a bovine whey protein in the blood of allergic people and observed that these molecules also react with the corresponding human protein. Does this same kind of cross-reaction exist for beta-casein? Wal and his colleagues tested the serum of twenty people allergic to the beta-casein of cow's milk, carefully measuring the concentration of immunoglobulin
E specific to bovine beta-casein and then verifying that these molecules also reacted with human beta-casein.
Human and bovine beta-caseins share 50% of their amino acids, hence the interest in studying their common domains. One such domain corresponds to an amino acid sequence that assumes a helical form in both human and bovine proteins. Another common domain contains the principal phosphorylation sites of beta-casein. Once the amino acids that make up this protein have been assembled, it is modified in particular by the addition of phosphate groups, a process known as phosphorylation. Such posttranslational chemical modifications influence the properties of the molecules by changing their electrical charge and altering their conformation. The inra-cea immunologists were able to show that the phosphorylation of this second domain affects the allergenic character of beta-casein.
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