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Some microbes have evolved ways to subvert or evade the innate immune system. The next line of host defense is the adaptive immune system, which is composed of lymphocytes (also called lymphoid cells) and their secreted factors (see Table 57–1).

A critical property of adaptive immunity is that the immune response is specifically tailored against different microbes. This is achieved by first generating an enormous number of diverse lymphocytes, each with a unique antigen specificity. Before they see their antigen, these lymphocytes are called naïve (Figure 59–1). How these cells function is closely linked to how they develop from stem cells, so in order to understand how lymphocytes work, it is first necessary to review lymphocyte development.


Development of naïve lymphocytes. Common lymphoid progenitors give rise to B-cell precursors, which develop into mature B lymphocytes in the bone marrow, and T-cell precursors, which leave the bone marrow and complete their development into mature CD4-positive and CD8-positive T cells in the thymus. Mature naïve lymphocytes migrate throughout the secondary lymphoid tissue surveying for antigen. CD = cluster of differentiation.


As described in Chapter 58, all white and red blood cells originate from stem cells (see Figure 58–1). The common lymphoid progenitor is a stem cell that gives rise to lymphocytes of the adaptive immune system, including B cells and T cells. The common lymphoid progenitor is also the source of innate lymphocytes, such as natural killer (NK) cells. The process by which common lymphoid progenitors develop into lymphocytes depends on cytokines, and mutations in cytokine signaling can cause severe combined immunodeficiency, a complete absence of mature lymphocytes (see Chapter 68). Table 59–1 compares various important features of B cells and T cells. These features will be discussed in detail in this and later chapters.

TABLE 59–1Comparison of T Cells and B Cells


All vertebrates produce enormously diverse pools of antigen ...

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