Both T cells and B cells originate in the bone marrow. Lymphocytes can travel throughout the body, using either the blood vessels or their own system of lymphatic vessels. Like small creeks that empty into larger and larger rivers, the lymphatic vessels feed into larger and larger channels. At the base of the neck they merge into a large duct, which discharges its contents into the blood stream. They can also be found in secondary lymphoid organs such as the spleen, lymph nodes, and other lymph tissues, including tonsils, the Peyer patches of the gut, and the respiratory and urinary tracts.
Lymphocytes can also accumulate in the connective and epithelial tissues throughout the body.
T cells contribute to the immune defenses in two major ways. Some direct and regulate the immune responses. Others are killer cells that attack cells that are infected or cancerous. Helper-Inducer T Cells, also called helper T cells are the commanders of the immune army, and serve as the central hub of immune operations. The chief responsibility of the helper T cells is to rouse the other defender cells into motion.