Porth's Pathophysiology, 9e

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UNIT IV Infection, Inflammation, and Immunity

Lymphoid Organs The central and peripheral lymphoid organs are respon- sible for the production, maturation, and storage of large numbers of immune system cells including the B and T lymphocytes. These organs and tissues are widely distrib- uted throughout the body and provide different, but often overlapping, functions (Fig. 13.12). The central lymphoid organs are comprised of the bone marrow and the thymus and are responsible for immune cell production and mat- uration. The tissues and cells of the peripheral lymphoid system store the cells of the immune system where they function to concentrate and process antigen as well as support cellular processes necessary for development of fully functioning, adaptive immune responses. The periph- eral lymphoid tissues are comprised of the lymph nodes, spleen, tonsils, appendix, Peyer patches in the intestine, and mucosa-­associated lymphoid tissues in the respiratory, ­gastrointestinal, and reproductive systems. Networks of lymph channels, blood vessels, and capillaries connect the lymphoid organs and transport immune cells, antigens, and cellular debris throughout the body.

Thymus The thymus is an elongated, bilobed structure located in the mediastinum above the heart and serves as a specialized immune system organ. Each lobe is surrounded by a connec- tive tissue capsule layer and is divided into lobules. The lob- ules can be divided into an outer cortex and a central medulla, which play different roles in the process of T-lymphocyte maturation. The outer cortex contains densely packed imma- ture T lymphocytes (thymocytes). The inner medulla is a less dense area of tissue that contains fewer but more histologically mature lymphocytes. The medulla is comprised of Hassall corpuscles but also stores DCs and macrophages (Fig. 13.13). The thymus is essential to the development of the immune system because it is responsible for the production of mature, immunocompetent T lymphocytes. The thymus is a fully developed organ at birth, weighing approximately 15 to 20 g. It is most active in the neonatal and preadoles- cent periods. At puberty, when the immune cells are well established in peripheral lymphoid tissues, the thymus begins to atrophy and is replaced by adipose tissue. Nevertheless, residual T-lymphocyte production continues throughout adult life. Precursor T (pre-T) cells enter the thymus as function- ally and phenotypically immature T cells. They then mature during different cycles and then move from the cortex to the medulla until they are released into the peripheral lymphoid tissues. Rapid cell division, maturation, and selection occur

Adenoid

Dead cell

Macrophage

Tonsil

Dividing thymocyte

Thymus

Bronchus- associated lymphoid tissue

Hassall corpuscles

Axillary lymph nodes

Spleen

Intestine

Inguinal lymph nodes

Peyer patches

Appendix

Bone marrow

Capsule

Epithelial cell

Dendritic cell

FIGURE 13.13  •  Structural features of the thymus gland. The ­thymus gland is divided into lobules containing an outer cortex densely packed with dividing thymocytes or premature T cells and an inner ­medulla that contains mature T lymphocytes, macrophages, dendritic cells, and ­Hassall corpuscles.

FIGURE 13.12  •  Central and peripheral lymphoid organs and tissues.