Primary Lymphoid Organs

Immature lymphocytes generated in hematopoiesis mature and become committed to a particular antigenic specificity within the primary lymphoid organs. Only after a lympho-

Flowcytometry Beads Coated With Antigens

FIGURE 2-12

Scanning electron micrograph of follicular dendritic cells showing long, beaded dendrites. The beads are coated with antigen-antibody complexes. The dendrites emanate from the cell body. [From A. K. Szakal et al, 1985, J. Immunol. 134:1353; © 1996 by American Association of Immunologists, reprinted with permission.]

FIGURE 2-12

Scanning electron micrograph of follicular dendritic cells showing long, beaded dendrites. The beads are coated with antigen-antibody complexes. The dendrites emanate from the cell body. [From A. K. Szakal et al, 1985, J. Immunol. 134:1353; © 1996 by American Association of Immunologists, reprinted with permission.]

Go to www.whfreeman.com/immunology Cells and Organs of the Immune System

Animation

Adenoids Tonsil

Thoracic duct lymphatic duct

Small intestine

Adenoids Tonsil

Thoracic duct lymphatic duct

Primary Lymphoid Organ

Small intestine

Bone marrow

Tissue lymphatics

FIGURE 2-13

The human lymphoid system. The primary organs (bone marrow and thymus) are shown in red; secondary organs and tissues, in blue. These structurally and functionally diverse lymphoid organs and tissues are interconnected by the blood vessels (not shown) and lymphatic vessels (purple) through which lymphocytes circulate. Only one bone is shown, but all major bones contain marrow and thus are part of the lymphoid system. [Adapted from H. Lodish et al., 1995, Molecular Cell Biology, 3rd ed., Scientific American Books.]

Bone marrow

Tissue lymphatics

FIGURE 2-13

The human lymphoid system. The primary organs (bone marrow and thymus) are shown in red; secondary organs and tissues, in blue. These structurally and functionally diverse lymphoid organs and tissues are interconnected by the blood vessels (not shown) and lymphatic vessels (purple) through which lymphocytes circulate. Only one bone is shown, but all major bones contain marrow and thus are part of the lymphoid system. [Adapted from H. Lodish et al., 1995, Molecular Cell Biology, 3rd ed., Scientific American Books.]

cyte has matured within a primary lymphoid organ is the cell immunocompetent (capable of mounting an immune response). T cells arise in the thymus, and in many mammals—humans and mice for example—B cells originate in bone marrow.

THYMUS

The thymus is the site of T-cell development and maturation. It is a flat, bilobed organ situated above the heart. Each lobe is surrounded by a capsule and is divided into lobules, which are separated from each other by strands of connective tissue called trabeculae. Each lobule is organized into two compartments: the outer compartment, or cortex, is densely packed with immature T cells, called thymocytes, whereas the inner compartment, or medulla, is sparsely populated with thymocytes.

Both the cortex and medulla of the thymus are crisscrossed by a three-dimensional stromal-cell network composed of epithelial cells, dendritic cells, and macrophages, which make up the framework of the organ and contribute to the growth and maturation of thymocytes. Many of these stromal cells interact physically with the developing thymocytes (Figure 2-14). Some thymic epithelial cells in the outer cortex, called nurse cells, have long membrane extensions that surround as many as 50 thymocytes, forming large multicellular complexes. Other cortical epithelial cells have long interconnecting cytoplasmic extensions that form a network and have been shown to interact with numerous thymocytes as they traverse the cortex.

The function of the thymus is to generate and select a repertoire of T cells that will protect the body from infection. As thymocytes develop, an enormous diversity of T-cell receptors is generated by a random process (see Chapter 9) that produces some T cells with receptors capable of recognizing antigen-MHC complexes. However, most of the T-cell receptors produced by this random process are incapable of recognizing antigen-MHC complexes and a small portion react with combinations of self antigen-MHC complexes. Using mechanisms that are discussed in Chapter 10, the thymus induces the death of those T cells that cannot recognize anti-gen-MHC complexes and those that react with self-antigen-MHC and pose a danger of causing autoimmune disease. More than 95% of all thymocytes die by apoptosis in the thymus without ever reaching maturity.

the thymus and immune function The role of the thymus in immune function can be studied in mice by examining the effects of neonatal thymectomy, a procedure in which the thymus is surgically removed from newborn mice. These thymectomized mice show a dramatic decrease in circulating lymphocytes of the T-cell lineage and an absence of cell-mediated immunity. Other evidence of the importance of the thymus comes from studies of a congenital birth defect in humans (DiGeorge's syndrome) and in certain mice (nude mice) in which the thymus fails to develop. In both cases, there is an absence of circulating T cells and of cell-mediated immunity and an increase in infectious disease.

Aging is accompanied by a decline in thymic function. This decline may play some role in the decline in immune function during aging in humans and mice. The thymus reaches its maximal size at puberty and then atrophies, with a significant decrease in both cortical and medullary cells and

Capsule

Trabecula Dividing thymocyte

Dead cell

Nurse cell

Cortical epithelial cell

Capsule

Dead cell

Nurse cell

Cortical epithelial cell

Thymus Cross Section

Hassall's corpuscles

Interdigitating dendritic cell Medullary epithelial cell

Diagrammatic cross section of a portion of the thymus, showing several lobules separated by connective tissue strands (trabeculae). The densely populated outer cortex is thought to contain many immature thymocytes (blue), which undergo rapid proliferation coupled with an enormous rate of cell death. Also present in the outer cortex are thymic nurse cells (gray), which are specialized epithelial cells with long membrane extensions that surround as many as 50 thymocytes. The medulla is sparsely populated and is thought to contain thymocytes that are more mature. During their

Blood vessel Macrophage

Hassall's corpuscles

Interdigitating dendritic cell Medullary epithelial cell

FIGURE 2-14

Diagrammatic cross section of a portion of the thymus, showing several lobules separated by connective tissue strands (trabeculae). The densely populated outer cortex is thought to contain many immature thymocytes (blue), which undergo rapid proliferation coupled with an enormous rate of cell death. Also present in the outer cortex are thymic nurse cells (gray), which are specialized epithelial cells with long membrane extensions that surround as many as 50 thymocytes. The medulla is sparsely populated and is thought to contain thymocytes that are more mature. During their stay within the thymus, thymocytes interact with various stromal cells, including cortical epithelial cells (light red), medullary epithelial cells (tan), interdigitating dendritic cells (purple), and macrophages (yellow). These cells produce thymic hormones and express high levels of class I and class II MHC molecules. Hassalls corpuscles, found in the medulla, contain concentric layers of degenerating epithelial cells. [Adapted, with permission, from W. van Ewijk, 1991, Annu. Rev. Immunol. 9:591, © 1991 by Annual Reviews.]

an increase in the total fat content of the organ. Whereas the average weight of the thymus is 70 g in infants, its age-dependent involution leaves an organ with an average weight of only 3 g in the elderly (Figure 2-15).

A number of experiments have been designed to look at the effect of age on the immune function of the thymus. In one experiment, the thymus from a 1-day-old or 33-month-old mouse was grafted into thymectomized adults. (For most laboratory mice, 33 months is very old.) Mice receiving the newborn thymus graft showed a significantly larger improvement in immune function than mice receiving the 33-month-old thymus.

BONE MARROW

In humans and mice, bone marrow is the site of B-cell origin and development. Arising from lymphoid progenitors, immature B cells proliferate and differentiate within the bone marrow, and stromal cells within the bone marrow interact directly with the B cells and secrete various cytokines that are required for development. Like thymic selection during T-cell maturation, a selection process within the bone marrow eliminates B cells with self-reactive antibody receptors. This process is explained in detail in Chapter 11. Bone marrow is not the site of B-cell development in all species. In birds, a lymphoid organ called the bursa of Fabricius, a lymphoid tissue associated with the gut, is the primary site of B-cell maturation. In mammals such as primates and rodents, there is no bursa and no single counterpart to it as a primary lym-phoid organ. In cattle and sheep, the primary lymphoid tissue hosting the maturation, proliferation, and diversification of B cells early in gestation is the fetal spleen. Later in gestation, this function is assumed by a patch of tissue embedded

Changes Thymus With Age

Birth 10

FIGURE 2-15

Birth 10

FIGURE 2-15

Changes in the thymus with age. The thymus decreases in size and cellularity after puberty.

in the wall of the intestine called the ileal Peyer's patch, which contains a large number (> 1010) B cells. The rabbit, too, uses gut-associated tissues such as the appendix as primary lym-phoid tissue for important steps in the proliferation and diversification of B cells.

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Responses

  • scott
    Why thymus is called primary lymphoid organ?
    8 years ago
  • bryan
    What is the primary lymphoid tissue and primary lymphoid organ?
    8 years ago
  • luwam tewolde
    Is thymus a primary or secondary lymphoid organ?
    8 years ago
  • carmen engelke
    What are all the lumph organs?
    7 years ago
  • Makda
    Which lymphoid organ may have a role in this decline?
    7 years ago
  • David Palen
    What are primary lymphoid organs?
    7 years ago
  • Justiina
    Which lymphoid organ may have a role in the decline of fighting infection?
    6 years ago
  • Jani
    Which tissues are considered to be primary lymphoid organ in sheep?
    2 years ago
  • Petra
    Which lymphoid tissue is considered primary lymphoid organ in sheep?
    2 years ago
  • Hamilcar
    What is the associated organs of primary lymphoid organ?
    1 year ago
  • VELI-MATTI
    What are primary lymphoid organs and tissue Give at least one example of each?
    1 year ago

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