The basis of an immune response is that of clonal recognition and expansion. When
the immune system first encounters an antigen, the relatively few lymphocytes which
have the complementary antigen receptor will respond (primary response) by undergoing
expansion and differentiation. In the event that the same antigen is encountered again, a
much larger population of the previously differentiated (primed) lymphocytes will be
available, thereby mounting a quicker and more effective secondary immune response.