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The lymphoid tissues can be divided into primary and secondary lymphoid organs. Primary lymphoid tissues are sites where lymphocytes develop from progenitor cells into functional and mature lymphocytes. The major primary lymphoid tissue is the marrow, the site where all lymphocyte progenitor cells reside and initially differentiate. This organ is discussed in Chap. 4. The other primary lymphoid tissue is the thymus, the site where progenitor cells derived from the marrow differentiate into mature thymus-derived cells. Secondary lymphoid tissues are sites where lymphocytes undergo additional maturation and interact with each other and with nonlymphoid cells to generate immune responses to antigens. These tissues include the spleen, lymph nodes, and mucosa-associated lymphoid tissues such as tonsils. The structure of these tissues provides insight into how the immune system discriminates between self-antigens and foreign antigens and develops the capacity to orchestrate a variety of specific and nonspecific defenses against invading pathogens.

Acronyms and Abbreviations

AIRE, autoimmune regulatory gene; APECED, autoimmune polyendocrinopathy-candidiasis- ectodermal dystrophy; C, capsule; CT, computed tomography; GALT, gastrointestinal-associated lymphoid tissue; ILC, Innate lymphoid cell; LN, lymphatic nodule; LTi, lymphoid tissue inducer; MALT, mucosa-associated lymphoid tissues; MHC, major histocompatibility complex; NK, natural killer cell; PALS, periarteriolar lymphoid sheath; PGA syndrome, polyglandular autoimmune syndrome; T, thymus-derived; TCR, T-cell receptor.


The thymus is the site for development of thymic-derived lymphocytes, or T cells. In this organ, developing T cells, called thymocytes, differentiate from lymphoid stem cells derived from the marrow into functional, mature T cells.1 It is here that T cells acquire their repertoire of specific antigen receptors to cope with the antigenic challenges received throughout one’s life span. After they have completed their maturation, the T cells leave the thymus and circulate in the blood and through secondary lymphoid tissues.


The thymus is located in the superior mediastinum, overlying, in order, the left brachiocephalic (or innominate) vein, the innominate artery, the left common carotid artery, and the trachea. It overlaps the upper limit of the pericardial sac below and extends into the neck beneath the upper anterior ribs. It receives its blood supply from the internal thoracic arteries. Venous blood from the thymus drains into the brachiocephalic and internal thoracic veins, which communicate above with the inferior thyroid veins.

Arising from the third and fourth brachial pouches as an epithelial organ populated by lymphoid cells and endoderm-derived thymic epithelial cells, the thymus develops at about the eighth week of gestation.2 The thymus increases in size through fetal and postnatal life and remains ample into puberty,3 when it weighs approximately 40 g. Thereafter, the size progressively decreases with aging as a consequence of thymic involution.4 The cause of thymic involution is likely in part caused by the influence of steroid hormones.5 Nonetheless, there is evidence that T lymphocytes continue to ...

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