Lymphocyte migration: an essential step in understanding the effects of vaccination.

When an antigen has passed the epithelial barrier of the skin or mucosal surfaces it has to be processed and presented by accessory cells to lymphocytes. These reactions take place in lymphoid organs, such as the regional lymph nodes, Peyer's patches and tonsils, but also in the spleen if the antigen entered the blood directly. The respective lymphocyte clone expands by proliferating, and primed lymphocytes of the B and T cell series emigrate from the lymphoid organs. The traffic of lymphocytes is regulated by the interaction of a series of adhesion molecules with endothelial cells and lymphocytes. Several earlier ideas, for instance one specific "homing receptor" for each organ and different receptors for B and T lymphocytes, or exclusive migratory routes for "memory" and "naive" lymphocytes, have had to be replaced by the concept of a much more complex, multistep, cascade-type reaction. Most migration routes show "preference" rather than "selectivity". The regulation of the entry of activated T and B lymphocytes into the parenchyma of non-lymphoid organs, e.g. the lamina propria of the gut, is not as well as understood as the entry into a lymph node. A further important aspect in lymphocyte traffic is the regulation of lymphocyte migration within the organs, including the interaction between lymphoid cells and the extracellular matrix. the basic mechanisms of lymphocyte migration have to be considered when the effects of vaccination procedures are interpreted.