Migration pathways and immunologic memory among T lymphocytes.

The lymphatic and circulatory systems are essential channels for the dissemination of memory cells throughout the body. However, the migration of naive and memory T cells through these channels is not random. Naive-type T cells preferentially migrate from blood to lymph nodes whereas memory T cells preferentially migrate to tissues, particularly those with a high exposure to antigen. The large-scaled migration of naive T cells through lymph nodes increases the likelihood of these T cells encountering a primary antigen, and brings them in contact with other players, particularly antigen presenting cells. On the other hand, the migration of memory T cells to tissues such as skin or gut mucosa serves to provide an immediate protection in an environment where antigen is likely to be re-encountered. The migration of memory T cells is further rationalized, in that phenotypically distinct subsets of memory T cells migrate to specific tissues. The migration of lymphocytes through the body is controlled by adhesion molecules on the surface of lymphocytes, which interact with receptors on the surface of endothelium, and it is the differential expression of these molecules which in part controls the different migration streams of T cells through the body.