B

lymphocyte lineage lymphocyte lineage

Thymus Mature naive T lymphocytes

FIGURE 2-5 Maturation of lymphocytes. Lymphocytes develop from bone marrow stem cells and mature in the generative lymphoid organs (bone marrow and thymus for B and T cells, respectively) and then circulate through the blood to secondary lymphoid organs (lymph nodes, spleen, regional lymphoid tissues such as mucosa-associated lymphoid tissues). Fully mature T cells leave the thymus, but immature B cells leave the bone marrow and complete their maturation in secondary lymphoid organs. Naive lymphocytes may respond to foreign antigens in these secondary lymphoid tissues or return by lymphatic drainage to the blood and recirculate through other secondary lymphoid organs.

Thymus Mature naive T lymphocytes

Mature T lymphocytes)

FIGURE 2-5 Maturation of lymphocytes. Lymphocytes develop from bone marrow stem cells and mature in the generative lymphoid organs (bone marrow and thymus for B and T cells, respectively) and then circulate through the blood to secondary lymphoid organs (lymph nodes, spleen, regional lymphoid tissues such as mucosa-associated lymphoid tissues). Fully mature T cells leave the thymus, but immature B cells leave the bone marrow and complete their maturation in secondary lymphoid organs. Naive lymphocytes may respond to foreign antigens in these secondary lymphoid tissues or return by lymphatic drainage to the blood and recirculate through other secondary lymphoid organs.

in lymphocyte development occur are called the generative lymphoid organs. These include the bone marrow, where precursors of all lymphocytes arise and B cells mature, and the thymus, where T cells mature (Fig. 2-5). We will discuss the processes of B and T lymphocyte maturation in much more detail in Chapter 8. These mature B and T cells are called naive lymphocytes. After activation by antigen, lymphocytes go through sequential changes in phenotype and functional capacity.

Populations of Lymphocytes Distinguished by History of Antigen Exposure

In adaptive immune responses, naive lymphocytes that emerge from the bone marrow or thymus migrate into peripheral lymphoid organs, where they are activated by antigens to proliferate and differentiate into effector and memory cells, some of which then migrate into tissues (Fig. 2-6). The activation of lymphocytes follows a series of sequential steps beginning with the synthesis of new proteins, such as cytokine receptors and cytokines, which are required for many of the subsequent changes. The naive cells then undergo proliferation, resulting in increased size of the antigen-specific clones, a process that is called clonal expansion. In some infections, the numbers of microbe-specific T cells may increase more than 50,000-fold, and the numbers of specific B cells may increase up to 5000-fold. This rapid clonal expansion of microbe-specific lymphocytes is needed to keep pace with the ability of microbes to rapidly replicate and expand in numbers. Concurrently with clonal expansion, antigen-stimulated lymphocytes differentiate into effector cells whose function is to eliminate the antigen. Some of the progeny of antigen-stimulated B and T lymphocytes differentiate into long-lived memory cells, whose function is to mediate rapid and enhanced (i.e., secondary) responses to subsequent exposures to antigens. Distinct populations of lymphocytes (naive, effector, and memory) are always present in various sites throughout the body, and these populations can be distinguished by several functional and phenotypic criteria (Table 2-3).

The details of lymphocyte activation and differentiation as well as the functions of each of these populations will be addressed later in this book. Here we summarize the phenotypic characteristics of each population.

Naive Lymphocytes

Naive lymphocytes are mature T or B cells that reside in the peripheral lymphoid organs and circulation and have never encountered foreign antigen. (The term naive refers to the idea that these cells are immunologically

Naive B cells

Secondary (peripheral) lymphoid organs

Naive B cells

Effector T lymphocytes and antibodies

Secondary (peripheral) lymphoid organs

FIGURE 2-6 The anatomy of lymphocyte activation. Naive T cells emerging from the thymus and immature B cells emerging from the bone marrow migrate into secondary lymphoid organs, including lymph nodes and spleen. In these locations, B cells complete their maturation; naive B and T cells activated by antigens differentiate into effector and memory lymphocytes. Some effector and memory lymphocytes migrate into peripheral tissue sites of infection. Antibodies secreted by effector B cells in lymph node, spleen, and bone marrow (not shown) enter the blood and are delivered to sites of infection.

Mucosa/ skin

Collection of antigens from tissues via lymph

Effector T lymphocytes and antibodies

Activation of lymphocytes and initiation of adaptive immune responses

Collection of antigens via blood

Migration of effector cells, and blood delivery of antibodies to site of infection

Entry of infectious agents and/or 1 environmental antigens

Antigen-presenting cell

Mucosa/ skin

Antigen-presenting cell

FIGURE 2-6 The anatomy of lymphocyte activation. Naive T cells emerging from the thymus and immature B cells emerging from the bone marrow migrate into secondary lymphoid organs, including lymph nodes and spleen. In these locations, B cells complete their maturation; naive B and T cells activated by antigens differentiate into effector and memory lymphocytes. Some effector and memory lymphocytes migrate into peripheral tissue sites of infection. Antibodies secreted by effector B cells in lymph node, spleen, and bone marrow (not shown) enter the blood and are delivered to sites of infection.

TABLE 2-3 Characteristics of Naive, Effector, and Memory Lymphocytes

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