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The innate immune system is often capable of containing and eradicating microbial invaders, but some microbes have evolved ways to subvert or evade innate immunity. 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).
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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 can aid host defense or can cause disease, it is first necessary to understand lymphocyte development.
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ORIGIN OF LYMPHOID CELLS
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As described in Chapter 58, all white and red blood cells originate from stem cells in the fetal liver and yolk sac during embryonic life and in the bone marrow after birth (see Figure 58–1). The common lymphoid progenitor is a type of 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 the genes encoding the receptors of these cytokines are often the cause of severe combined immunodeficiency, a complete absence of mature lymphocytes (see Chapter 68).
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The ratio of T cells to B cells is approximately 3:1. Figure 59–1 describes the origin of B cells and two of the main types of T cells. Often T cells are named by markers we can detect on their cell surface, called “cluster of differentiation” (CD) markers: helper T cells are CD4-positive (CD4+), whereas cytotoxic T cells are CD8-positive (CD8+). 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.
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