Definition of an allergen (immunobiology)

A variety of terms are used to define the substance that stimulates an atopic reaction. Which words are used depends on the terms chosen to denote the sensitivity (1). In the context of a general immunological reaction, the triggering substance is called an antigen. In modern usage, an antigen is any substance that induces a state of sensitivity and/or resistance as a result of coming into contact with appropriate tissues of an animal body (2). The observed sequence-exposure to substance, latency period, manifestation of substance-specific sensitization upon reexposure to substance-is characteristic of immunological memory, and indicates that B cells and/or T cells are involved.