It was not until the mid-1920's, that a second significant event occurred. This positive skin test for allergies would show itself most prominently in patients with hay fever, asthma, chronic rhinitis, hives and eczema. However, the precise biological reason for the reaction continued to remain a mystery. It was not until the Sixties, when an important discovery occurred which provided long-awaited scientific support for the classical allergy theory and removed any doubts about the relationship of the immune system with allergies. This allergen is the substance which causes us to have an abnormal immunological response.
Allergens tend to be protein molecules.
Interestingly enough, the immune system only detects particles of a certain size as potential troublemakers and protein molecules are just the right size. In a small number of cases, the body actually responds to molecules other than proteins. These molecules, which are generally much smaller, are called haptens. By combining with protein molecules, haptens form larger complexes which can then be detected by the immune system . The allergen is detected by the B cells.