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The organs in which lymphocyte maturation, differentiation, and proliferation take place are divided into two categories: primary and secondary organs. The primary or central lymphoid organs are those in which the maturation of B and T lymphocytes into antigen‐recognizing lymphocytes occurs. In other words, these are the organs where gene rearrangements occur to generate functional antigen‐specific BCRs and TCRs expressed by B and T cells, respectively. Mature B cells differentiate to fully mature cells within the bone marrow. Historically, the term “B cell” is derived from developmental studies in birds that demonstrated that antibody‐forming lymphocytes differentiate within an organ unique to birds called the bursa of Fabricius (hence, “B” for bursa). In contrast, T cells differentiate only partially within the bone marrow. Precursor cells destined to become mature T cells undergo final maturation within the thymus gland (hence, “T” for thymus). Histological characteristics of the thymus are discussed in the next section.

Mature B and T cells migrate through the bloodstream and lymphatic system to the peripheral lymphoid tissues, including the lymph nodes and spleen. Collectively, these are referred to as the secondary lymphoid organs and this is where antigen‐driven activation (proliferation and differentiation) of B and T cells takes place (Figure 2.2). Histological properties of the secondary lymphoid organs are also presented below.

Figure 2.1. Lymphoid fluid draining interstitial spaces.

Source: © John Wiley & Sons, Inc.

Figure 2.2. Distribution of lymphoid tissues in the body.