Antigen
Antigens are "targeted" by antibodies. Each antibody is specifically produced by the immune system to match an antigen after cells in the immune system come into "contact" with it; this allows a precise identification or matching of the antigen and the initiation of a tailored response. The antibody is said to "match" the antigen in the sense that it can bind to it due to an adaptation in a region of the antibody; because of this, many different antibodies are produced, each able to bind a different antigen while sharing the same basic structure. In most cases, an adapted antibody can only react to and bind one specific antigen; in some instances, however, antibodies may cross-react and bind more than one antigen.
Thymus
Allergy results from an inappropriate and excessive immune response to common antigens. Substances that trigger an allergic response are called allergens. Allergies involve mainly IgE, antibodies, and histamine. Mast cells release the histamine. Sometimes an allergen may cause a sudden and severe, possibly fatal reaction in a sensitive individual; this is called anaphylaxis.