Allergenicity and Hypersensitivity

Food allergies are adverse reactions to a protein or glycoprotein in food that elicits a heightened response of the immune system in some people. Among several types of immunologic responses causing food allergies, the most common type of reaction is mediated by allergen-specific immunoglobulin E (IgE) antibodies. IgE-mediated reactions are known as immediate or acute hypersensitivity reactions because symptoms occur within minutes to several hours after ingestion of the allergenic food. Food allergies also include delayed hypersensitivity reactions whose mechanisms are less clear. These include cell-mediated reactions where the onset of symptoms occurs more than eight hours after ingestion of the allergenic food. In the United States, the prevalence of food allergies is 1.5 percent of the general population, and 5 percent of children under three years of age (Sampson, 1997). The prevalence of these types of reactions in infants remains uncertain, but cases have been well documented (FAO, 2001). Many children outgrow their food allergies (Sampson, 1997; Taylor et al., 1999). There are eight foods or food groups that account for more than 90 percent of the food allergies in the United States. These include cow’s milk, eggs, fish, crustaceans, peanuts, soybeans, tree nuts, and wheat (Taylor et al., 1999). However, more than 160 other foods have been identified as causing food allergies (Hefle et al., 1996).

The genetic engineering of animals intended for use as food will involve the expression of new proteins in animals; hence the safety, including the potential allergenicity of the newly introduced proteins, will have to be assessed. While most known allergens are proteins, only a few of the innumerable proteins found in foods are allergenic under typical circumstances of exposure (Taylor and Hefle, 2001). While the common sources of food allergens have been identified and characterized, many others are less known and poorly understood. If the new protein originates from a known allergenic source or its amino acid sequence is similar to that of a known allergen, the protein can be tested to determine whether it causes a reaction with sera from individuals with known food allergies. However, the potential allergenicity of a protein can be reasonably assessed only when the protein is known to trigger an immune response in sensitive subjects. By contrast, the potential allergenicity of a protein of unknown allergenicity cannot easily be predicted, as no immunosera of allergic subjects are available (Mendieta et al., 1997). A more difficult issue arises when a new protein comes from a source that historically is not a human food. Assessing the potential allergenicity of transferred proteins remains one of the most difficult aspects in the overall safety assessment of transgenic foods. An adequate allergenicity assessment will require an understanding of several factors, including the source of the transferred protein, its level of expression, the physical and chemical properties of the protein, and any structural similarities to known allergens. No single factor can be considered definitive,

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