Allergens.

What makes an antigen an allergen? This question has been at the core of much research regarding the basic mechanisms of allergy as well as the development of diagnostic and therapeutic allergen products. Great strides have been made since Kjell Aas asked this question in his classic review in Allergy (1), but we appear no closer to the answer. The mechanisms leading to allergic reactions have been well described in recent years and our understanding of it at the molecular level has greatly improved. Conversely, the molecular structures responsible for these allergic reactions are not as well understood. Techniques for the isolation, identification, and epitope mapping of allergens have greatly enhanced our ability to test old and new ideas about what makes an antigen an allergen. We continue to search for "major allergens," and when we identify them, we hope to delineate "allergenic determinants" and "immunodominant epitopes." Even now, unique structural motifs or determinants responsible for allergenicity have not yet been identified, and we will probably never find them.