Allergy is a hypersensitivity disorder of the immune system, where our body hyper reacts to a normally harmless substance. People who have allergies are often sensitive to more than one substance. Going by that definition any substance is a potential allergen.
Allergy is a Type I hypersensitivity reaction, immunologic in nature, occurring within minutes after the combination of antigen and antibody bound to a mast cell in an individual previously sensitized to the antigen. It could be a systemic or localized reaction depending on the antigen (allergen) involved.
Most allergic hypersensitive reactions are a topic in nature. Atopy is a personal and/or familial tendency, usually in childhood or adolescence, to become sensitized and produce IgE antibodies in response to ordinary exposure to allergens, usually proteins. As a consequence these individuals can develop typical symptoms of asthma, rhino conjunctivitis or eczema.
Allergen exposure is a prerequisite for sensitization in a topic individuals. IgE antibodies can be detected very early in the blood, even before symptoms become evident; hence IgE sensitization is a “risk marker” for developing allergy.