The lymphocytes and APCs for adaptive immunity are distributed throughout the body in the blood, lymph, and epithelial and connective tissues. Lymphocytes are formed initially in primary lymphoid organs (the thymus and bone marrow), but most lymphocyte activation and proliferation occur in secondary lymphoid organs (the lymph nodes, the spleen, and diffuse lymphoid tissue found in the mucosa of the digestive system, including 14 The Immune System & Lymphoid Organs INNATE & ADAPTIVE IMMUNITY 267 CYTOKINES 269 ANTIGENS & ANTIBODIES 270 Classes of Antibodies 270 Actions of Antibodies 271 ANTIGEN PRESENTATION 271 CELLS OF ADAPTIVE IMMUNITY 273 Antigen-Presenting Cells 273 Lymphocytes 273 THYMUS 276 Role of the Thymus in T-Cell Maturation & Selection 279 MUCOSA-ASSOCIATED LYMPHOID TISSUE 281 LYMPH NODES 282 Role of Lymph Nodes in the Immune Response 284 SPLEEN 286 Functions of Splenic White & Red Pulp 286 SUMMARY OF KEY POINTS 293 ASSESS YOUR KNOWLEDGE 294 CHAPTER the tonsils, Peyer patches, and appendix). The immune cells located diffusely in the digestive, respiratory, or urogenital mucosae comprise what is collectively known as mucosa-associated lymphoid tissue (MALT). Proliferating B lymphocytes in the secondary structures of MALT are arranged in small spherical lymphoid nodules