Acquired Immunity
The acquired immunity is initiated by the recognition of a foreign antigen and represents the response of the immune system to this recognition. This component of immune system is highly adaptive and exhibits the following four characteristic features:
(1) antigen specificity,
(2) diversity,
(3) immunologic memory, and
(4) recognition of self from nonself antigens.

3. Once the immune system has recognized and responded to an antigen, it is able to respond to this antigen more strongly following a subsequent exponsure. This constitutes immunologic memory, and is the basis for long-term immunity to many pathogens.
4. The immune system normally responds only to foreign antigens. This ability to recognize self antigens from nonself ones is critical for normal function of the immune system. Sometimes this feature may fail and give rise to autoimmune diseases.