Anatomy And Functions Of Lymphoid Tissues

To optimize the cellular interactions necessary for antigen recognition and lymphocyte activation in adaptive immune responses, lymphocytes and APCs are localized and concentrated in anatomically defined tissues or organs, which are also the sites where foreign antigens are transported and concentrated. Such anatomic com-partmentalization is not fixed because, as we will discuss in Chapter 3, many lymphocytes recirculate and constantly exchange between the circulation and the tissues.

Lymphoid tissues are classified as generative organs, also called primary or central lymphoid organs, where lymphocytes first express antigen receptors and attain phenotypic and functional maturity, and as peripheral organs, also called secondary lymphoid organs, where lymphocyte responses to foreign antigens are initiated and develop (see Fig. 2-5). Included in the generative lymphoid organs of adult mammals are the bone marrow and the thymus for B cells and T cells, respectively. B lymphocytes partially mature in the bone marrow, enter the circulation, and then populate peripheral lymphoid organs, including spleen and lymph nodes, where they complete their maturation. T lymphocytes mature completely in the thymus, then enter the circulation and populate peripheral lymphoid organs and tissues. Two important functions shared by the generative organs are to provide growth factors and other molecular signals needed for lymphocyte maturation and to present self antigens for recognition and selection of maturing lymphocytes (see Chapter 8). The peripheral lymphoid organs and tissues include the lymph nodes, spleen, cutaneous immune system, and mucosal immune system. In addition, poorly defined aggregates of lymphocytes are found in connective tissue and in virtually all organs except the central nervous system. All peripheral lym-phoid organs also share common functions, including the delivery of antigens and responding naive lymphocytes to the same location so that adaptive immune responses can be initiated and the anatomic segregation of B and T lymphocytes except for specific times when they need to interact.

Bone Marrow

The bone marrow is the site of generation of most mature circulating blood cells, including red cells, granulocytes, and monocytes, and the site of early events in B cell maturation. The generation of all blood cells, called hematopoiesis (Fig. 2-9), occurs initially, during fetal development, in blood islands of the yolk sac and the para-aortic mesenchyme, then shifts to the liver between the third and fourth months of gestation, and gradually shifts again to the bone marrow. At birth, hematopoiesis takes place mainly in the bones throughout the skeleton, but it becomes restricted increasingly to the marrow of the flat bones so that by puberty, hematopoiesis occurs mostly in the sternum, vertebrae, iliac bones, and ribs. The red marrow that is found in these bones consists of a sponge-like reticular framework located between long trabeculae. The spaces in this framework contain a network of blood-filled sinusoids lined by endothelial cells attached to a discontinuous basement membrane. Outside the sinusoids are clusters of the precursors of blood cells in various stages of development as well as mature fat cells. The blood cell precursors mature and migrate through the sinusoidal basement membrane and between endothelial cells to enter the vascular circulation. When the bone marrow is injured or when an exceptional demand for production of new blood cells occurs, the liver and spleen often become sites of extra-medullary hematopoiesis.

Red cells, granulocytes, monocytes, dendritic cells, platelets, B and T lymphocytes, and NK cells all originate from a common hematopoietic stem cell (HSC) in the bone marrow (see Fig. 2-9). HSCs are pluripotent, meaning that a single HSC can generate all different types of mature blood cells. HSCs are also self-renewing because each time they divide, at least one daughter cell maintains the properties of a stem cell while the other can differentiate along a particular lineage (called asymmetric division). HSCs can be identified by the presence of surface markers, including the proteins CD34 and c-Kit, and the absence of lineage-specific markers. HSCs are maintained within specialized microscopic anatomic niches in the marrow. In these locations, nonhematopoi-etic stromal cells provide contact-dependent signals and soluble factors required for continuous self-renewing

Stem cells

Multipotent progenitors

Committed precursors

Common lymphoid progenitor

Common lymphoid progenitor

Lymphopoiesis

Myelopoiesis

Lymphopoiesis

Thrombo poietin, IL-11 ' T

Myelopoiesis

Thrombo poietin, IL-11 ' T

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