Allergenicity of an antigen in terms of Type-1 allergy has always been described as the ability of a given protein to bind to specific IgE antibodies and the potency of these allergens to crosslink cellbound IgE to induce mediator release like histamine in basophils or mast cells. Whether the ability of allergens to induce a primary sensitization by switching B-cells to produce specific IgE antibodies is linked to a special feature of the allergens is unknown. In this context it is argued that the function of an allergen itself contributes to the allergenicity either by representing a specific structure or by influencing the process of presentation. Recently evidence was presented that cysteine protease Der p 1, the major house dust mite allergen, increases the permeability of bronchial epithelium for albumin by its functional ability of protein cleavage and may thus facilitate its own penetration (1). Furthermore enzymatically intact phospholipase Al was able to induce an IgE mediated immune response in an animal model whereas the inactive mutant did not lead to IgE but IgGl production (2). Finally it was proposed that a number of allergens with high homology to protease inhibitors, which can bind directly to cell membranes of antigen presenting cells, influence their endocytosis by being protease inhibitors (3).