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The secondary lymphoid organs have two major functions: (1) they are highly efficient in trapping and concentrating foreign substances, and (2) they are the main sites of production of antibodies and the induction of antigen‐specific T lymphocytes. In other words, they are where immune responses are initiated.

Figure 2.3. (A) Cellular organization of the thymus.

Source: Rosen FS and Geha RS (2007). Reproduced with permission of Taylor & Francis.

(B) Section of an adolescent thymus showing capsule, septa, cortex, medulla, and Hassall’s corpuscle.

Source: Photograph by Dr John Lewis, SUNY Downstate College of Medicine, New York.

The major secondary lymphoid organs are the spleen and the lymph nodes. In addition, tonsils, appendix, clusters of lymphocytes distributed in the lining of the small intestine (Peyer’s patches), and lymphoid aggregates spread throughout mucosal tissue are considered secondary lymphoid organs. These secondary lymphoid organs are found in various areas of the body, such as the linings of the digestive tract, in the respiratory and genitourinary tracts, in the conjunctiva, and in the salivary glands, where mature lymphocytes interact with antigen and undergo activation. These mucosal secondary lymphoid organs have been given the name mucosa‐associated lymphoid tissue (MALT). Those lymphoid tissues associated with the gut are gut‐associated lymphoid tissue (GALT); those associated with the bronchial tree are termed bronchus‐associated lymphoid tissue (BALT).