What establishes a protein as an allergen?

There is little known about the factors that determine the allergenicity of food proteins. Apparently, the ability of a food protein to induce an alle rgic response requires its presence in substantial amounts in the food supp ly, its durability during food processing, and its resistance to digestion in the gastrointestinal tract. In addition to the mode and degree of exposu re, structural characteristics appear to play an important, role for the ca pacity of a protein to modulate the immune response towards allergic reacti ons. Until now, however, there has been no indication for common structural characteristics of linear T cell or linear IgE (B cell) epitopes and the k nowledge of structural characteristics of conformational IgE binding sites is very limited. Experimental data point only to certain surface areas of a llergenic proteins which are important for IgE binding. Therefore, it is no t possible to suggest any structural motif or conformational sequence patte rn common to all allergenic proteins. Furthermore, glycosylation appears no t to be a common critical determinant of allergenicity since food allergens comprise both glycoproteins and nonglycosylated proteins. Based on the few published three-dimensional structures of allergenic proteins including fo od proteins, one unifying feature of allergens appears to be their spherica l shape. The three-dimensional structures of many more allergens have to be determined, however, to allow for a better understanding of the molecular basis of allergenicity. Most recently, new ideas have been introduced as to why certain biochemical or biologic functions such as enzymatic activities may predispose a protein to become an allergen. Proteolytically active all ergens have been demonstrated to irritate the human mucosal surface, to enh ance their own transmucosal uptake, and to augment IgE production. Therefor e, the functional activity of some allergens may play a role among other fa ctors in the process of sensitization and allergic responses. (C) 2001 Else vier Science B.V. All rights reserved.