Lymphocytes develop from lymphoid progenitor cells in the thymus and bone marrow (yellow), which are therefore called central (or primary) lymphoid organs. The newly formed lymphocytes migrate from these primary organs to peripheral (or secondary) lymphoid organs, where they can react with foreign antigen. Only some of the peripheral lymphoid organs (blue) and lymphatic vessels (green) are shown; many lymphocytes, for example, are found in the skin and respiratory tract. As we discuss later, the lymphatic vessels ultimately empty into the bloodstream (not shown).
Figure 24–13 The development of b and T cells. The central lymphoid organs, where lymphocytes develop from lymphoid progenitor cells, are labeled in yellow boxes. The lymphoid progenitor cells develop from multipotent hematopoietic stem cells in the bone marrow. Some lymphoid progenitor cells develop locally in the bone marrow into immature B cells, while others migrate to the thymus (via the bloodstream) where they develop into thymocytes (developing T cells). Foreign antigens activate B cells and T cells mainly in peripheral lymphoid organs, such as lymph nodes or the spleen.
hematopoietic system, including erythrocytes, leukocytes, and platelets (see Figure 22–32).
Because they are sites where lymphocytes develop from lymphoid progenitor cells, the thymus and bone marrow are referred to as central (primary) lymphoid organs (see Figure 24–12). As we discuss later, most B and T cells die in the central lymphoid organs soon after they develop, without ever functioning. Others, however, mature and migrate via the blood to the peripheral (secondary) lymphoid organs—mainly the lymph nodes, spleen, and epithelium-associated lymphoid tissues in the gastrointestinal tract, respiratory tract, and skin. It is in these peripheral lymphoid organs that foreign antigens activate B and T cells (see Figure 24–13).
B and T cells become morphologically distinguishable from each other only after antigen has activated them: resting B and T cells look very similar, even in an electron microscope (Figure 24–14A). After activation by an antigen, both proliferate and mature into effector cells. Effector B cells secrete antibodies; in their most mature form, called plasma cells, they are filled with an extensive rough endoplasmic reticulum that is busily making antibodies (Figure 24–14B). In contrast, effector T cells (Figure 24–14C) contain very little endoplasmic reticulum and secrete a variety of cytokines rather than antibodies. Whereas B-cell-derived antibodies are widely distributed by the bloodstream, T-cell-derived cytokines mainly act locally on neighboring cells, although some are carried via the blood and act on distant host cells.
The most remarkable feature of the adaptive immune system is that it can respond to millions of different foreign antigens in a highly specific way. Human B cells, for example, collectively, can make more than 1012 different antibody molecules that react specifically with the antigen that induced their production. How can B cells and T cells respond specifically to such an enormous diversity of foreign antigens? The answer for both B and T cells is the same. As each lymphocyte develops in a central lymphoid organ, it becomes committed to react with a particular antigen before ever being exposed to the antigen. It expresses this commitment in the form of cell-surface receptors that specifically bind the antigen. When a lymphocyte encounters its antigen in a peripheral lymphoid organ, the binding of the antigen to the receptors (with help from co-stimulatory signals, discussed later)
Figure 24–14 electron micrographs of resting and effector lymphocytes.