The lymphoid system consists of organs that house 1) T and B cells that are responsible for antigen-specific immunity and 2) NK cells that are innately cytotoxic to tumor cells and cells expressing foreign antigens. The system is divided into a) central lymphoid organs, the thymus and bone marrow, and b) peripheral lymphoid organs including lymph nodes, spleen, and the mucosa/submucosa of the respiratory and alimentary tracts (Fig. 1-1). Lymphocytes are one of the principal leukocytes found in these organs. There are three major types of lymphocytes (T, B, and NK) that have distinctive surface markers and functions (see sections on T cells, B cells, and NK cells (Fig. 1-3 and Table 1-1). Furthermore, the T and B cells in the lymph nodes are confined to discrete zones (Fig. 1-4).

Figure 1-3

Principal surface markers of lymphocyte populations. Molecules that serve as receptors are shown in bold type.

Molecules that serve as receptors are shown in bold type.

Table 1-1

Major Features and Functions of Mononuclear Leukocytes.

Figure 1-4

Lymph node. Discrete B and T cell zones are found.