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Structure of food allergens in relation to allergenicity

The three-dimensional structure of a substantial number of allergens has been solved or can be predicted based on sequence homology with proteins of known structure. These structures can often be classified into a few structural families with common folds (1. ). Examples of four of the structural families that are particularly relevant from the point of view of food allergy are: 1. alpha-helical proteins: napins (52S albumins from seeds) and non-specific lipid transfer proteins (LTPs); 2. largely beta-sheet proteins with a prominent helix in close contact: lipocalins, profilins and Bet v 1-related proteins; 3. (alpha+beta)-structures, in which the alphaand beta-structural elements are not intimately associated: lactalbumin; and 4. serpins: (SERine Protease INhibitors) ovalbumin. Actually, ovalbumin is not itself a protease inhibitor, but is stucturally clearly related to this family of rather complex proteins. In addition to the 3D-structures of intact proteins, the structure of the sugar side chain that is responsible for crossreactions among foods from vegetable and invertebrate origin (CCD: cross-reactive carbohydrate determinant (2. )) is of interest. This 3D structure is well described by Elgavish and Shaanan (3. ). As discussed in more detail elsewhere (1. , 4. ), not all these food components are ‘‘complete’’ food allergens, i.e. allergens that are not only able to stimulate the immune system to produce IgE antibodies, but also to activate sensitised mast cells to release mediators. In the above-mentioned list, napins, LTPs, lactoglobulin (a lipocalin), lactalbumin and ovalbumin are complete food allergens, whereas Bet v 1-related proteins, profilins and CCD-epitopes are usually incomplete or secondary food allergens. These incomplete food allergens are relevant for food allergy only because of their cross-reactivity, often with pollen allergens. In this paper we will focus on three groups of proteins that are connected with milk allergy: the lysozyme family (with a-lactalbumin as example), the lipocalin family (with b-lactoglobulin as example) and serum albumins. We will use these examples in relation to the ongoing discussion on the implications in general of exposure to related proteins (either endogenously produced or entering the body via mucosal contact) on the immune repertoire and immune reactivity (5. , 6. ).