Both Types of Acquired Immunity Are Initiated by Antigens
Because acquired immunity does not develop until after invasion by a foreign organism or toxin, it is clear that the body must have some mechanism for recognizing this invasion. Each toxin or each type of organism almost always contains one or more specific chemical compounds in its makeup that are different from all other compounds. In general, these are proteins or large polysaccharides, and it is they that initiate the acquired immunity. These substances are called antigens (antibody generations). For a substance to be antigenic, it usually must have a high molecular weight, 8000 or greater. Furthermore, the process of antigenicity usually depends on regularly recurring molecular groups, called epitopes, on
the surface of the large molecule. This also explains why proteins and large polysaccharides are almost always antigenic, because both of these have this stereochemical characteristic