Both innate and adaptive immunity depend on the ability of the immune system to distinguish between self and non-self molecules. In immunology, "self" molecules are those components of an organism's body that can be distinguished from foreign substances by the immune system. Conversely, "non-self" molecules are those recognized as foreign molecules. One class of non-self molecules are called antigens (short for "anti"body "gen"erators) and are defined as substances that bind to specific immune receptors and elicit an immune response.