<P> A native antigen is an antigen that is not yet processed by an APC to smaller parts . T cells cannot bind native antigens, but require that they be processed by APCs, whereas B cells can be activated by native ones . </P> <P> Antigenic specificity is the ability of the host cells to recognize an antigen specifically as a unique molecular entity and distinguish it from another with exquisite precision . Antigen specificity is due primarily to the side - chain conformations of the antigen . It is measurable and need not be linear or of a rate - limited step or equation . </P>

Immunity resulting from natural exposure to an antigen in the environment is called