Some people are allergic to many offending substances, others to few and still others to none. What a person is allergic to is determined by many factors, including the environment, the history of exposures, the person's genes and the way the immune system works.
The skin, the respiratory tract, and the digestive tract are the organs that interact with the external environment. They are also the sites most affected by allergic disorders. The immune system protects the individual against foreign substances and microorganisms. Harmful "non-self" things are eliminated and beneficial "self" things are allowed to remain in place. The individual does not need to respond to most of the environmental substances to which he is exposed. Allergic diseases are caused by the immune system generating needless inflammatory responses to substances that otherwise might be harmless.
An antigen is a substance that generates an immune response. If this response is allergic, the inciting antigen may also be called an allergen. Each allergen has a unique molecular structure. The immune system uses the "blueprint" of this structure to make special proteins called antibodies that fit their corresponding allergens very specifically in a lock-and-key fashion. The antibodies that participate in the allergic response belong to the IgE class. Each IgE antibody is restricted to one allergen, but an individual may have many distinct antibodies that account for multiple allergic susceptibilities.