ABSTRACT
Lymphocytes produced in the primary lymphoid organs, thymus (T cells), and bone marrow (B cells), migrate via the bloodstream to the secondary lymphoid organs or tissues where they carry out their function. Since these cells have not yet encountered antigen, they are called “naive cells” and do not remain in one secondary lymphoid organ, but continue to recirculate around the body until they recognize their specific antigen (Figure 1). They enter the lymph nodes via the high endothelial venules (HEVs) and if they are not activated there, they pass via efferent lymphatic vessels into the thoracic duct and hence back into the bloodstream. Both memory and naive cells recirculate through the lymphoid tissues.
