Lymphocyte Origin, Development, and Residence

Just as all lymphocytes are originally derived from pluripotent stem cells that subsequently became lymphoid stem cells in the bone marrow (see Fig. 15-1), the three general classes of lym­phocytes (B cells, T cells, and NK cells) are all derived from lymphoid stem cells.

The differentiation of lymphoid stem cells to the types of mature lymphocytes begins early during embryonic development. Cells destined to become T cells leave the bone marrow and travel to the thymus, where secretions called thymic hormones act on them. These are not classic hormones, for they are local to the thymus, and lymphocytes must come to the thymus to be acted on. The thymic hormones guide the further development of the lympho­cytes to become T cells. As part of the develop­ment in the thymus, only T cells capable of recognizing foreign antigens are selected to survive. The selected T cells that leave the thymus populate lymphoid structures through­out the body, including tonsils, lymph nodes, the spleen, and collections of lymphocytes in the intestinal wall. These structures are respon­sible for producing T cells in adult animals (discussed later).

Lymphoid stem cells destined to become B cells also undergo differentiation during embry­onic development. In birds, lymphoid cells destined to become B cells leave the bone marrow and travel to the bursa of Fabricius, which is similar to the thymus but associated with the intestinal tract. Here the cells undergo maturation and selection similar to that which happens to pre-T cells in the thymus. Mature B cells leaving the bursa in birds also populate peripheral lymphoid structures, and it is these structures that produce B cells in adult birds. An organ that is the functional equivalent to the bursa has not been identified in mammals. It is thought that B cells mature in the bone marrow in mammals and leave the marrow to populate the peripheral lymphoid structures. As with T cells, the peripheral lymphoid structures produce B cells in adult animals. The develop­ment of NK cells probably occurs in a manner similar to that for B cells.