An allergen is an antigen, usually a protein, that is able to bind to a specific IgE and trigger an allergic reaction. Proteins account for more than half of the dry mass of most cells. Every organic substance is a potential allergen source and may contain a multitude of proteins with varying properties. 

 

A peanut, for instance, is not an individual allergen in itself, but sensitization may occur to any of the proteins it contains (see Figure). Since the word “allergen” is often used misleadingly in reference to an entire allergen source, individual allergens within an allergen source are called allergen components so as to avoid confusion. 

 

An allergen source contains proteins of greater and lesser importance in terms of allergic sensitization and symptoms. Allergens to which more than 50% of patients with the allergy are sensitized are usually classified as major allergens, while others are classified as minor allergens. The most important sensitizing allergen of an allergen source is usually a major allergen, and is often linked to clinically relevant symptoms as well. A major allergen is often quite specific for the allergen source.