These two subsets apparently circulate around the body by different pathways, and mediate the memory response in different ways. The results provide a new insight into the memory response, one that is relevant to basic immunology, as well as to vaccination strategies and the development of immunosuppressive therapies. The immune system responds slowly to newly encountered pathogens owing to the relative lack of antigen-specific cells. New pathogens must first be transported to nearby lymphoid tissue, where the primary immune response develops and where naive lymphocytes and antigen-presenting cells interact. This has been an elusive goal.
T cells can respond vigorously in vitro to previously encountered antigens.
The rationale is that a naive T cell has such a low probability of seeing its specific antigen that it needs to travel through the lymph nodes, which are designed for massive migration of lymphocytes and antigen sampling 2 . In contrast, memory or effector cells migrate mainly to peripheral tissues, providing protection at sites vulnerable to challenge by pathogens. But the problem with this system was that both memory and effector cells express CD45RO, so there was still no way to distinguish between the two. The CCR7 - subset has many characteristics of effector cells. So, expression of CCR7, in conjunction with CD45RO, distinguishes memory cells from effector cells.