rel="nofollow" href="#ulink_ff60c404-5abf-531d-8d08-dcdb4cdf70e3">Figure 2.2. Distribution of lymphoid tissues in the body.
Thymus Gland.
The thymus gland is a bilobed structure, derived from the endoderm of the third and fourth pharyngeal pouches. During fetal development, the size of the thymus increases. The growth continues until puberty. Thereafter, the thymus undergoes atrophy with aging.
The thymus is a lymphoepithelial organ and consists of epithelial cells organized into cortical (outer) and medullary (central) areas that are infiltrated with lymphoid cells (thymocytes) (Figure 2.3A). The cortex is densely populated with lymphocytes of various sizes, most of which are immature, and scattered macrophages involved in clearing apoptotic thymocytes. Figure 2.3B shows a section through normal human thymus tissue.
Secondary Lymphoid Organs.
The secondary lymphoid organs have two major functions: (1) they are highly efficient in trapping and concentrating foreign substances, and (2) they are the main sites of production of antibodies and the induction of antigen‐specific T lymphocytes. In other words, they are where immune responses are initiated.
Figure 2.3. (A) Cellular organization of the thymus.
Source: Rosen FS and Geha RS (2007). Reproduced with permission of Taylor & Francis.
(B) Section of an adolescent thymus showing capsule, septa, cortex, medulla, and Hassall’s corpuscle.
Source: Photograph by Dr John Lewis, SUNY Downstate College of Medicine, New York.
The major secondary lymphoid organs are the spleen and the lymph nodes. In addition, tonsils, appendix, clusters of lymphocytes distributed in the lining of the small intestine (Peyer’s patches), and lymphoid aggregates spread throughout mucosal tissue are considered secondary lymphoid organs. These secondary lymphoid organs are found in various areas of the body, such as the linings of the digestive tract, in the respiratory and genitourinary tracts, in the conjunctiva, and in the salivary glands, where mature lymphocytes interact with antigen and undergo activation. These mucosal secondary lymphoid organs have been given the name mucosa‐associated lymphoid tissue (MALT). Those lymphoid tissues associated with the gut are gut‐associated lymphoid tissue (GALT); those associated with the bronchial tree are termed bronchus‐associated lymphoid tissue (BALT).
The Spleen
The spleen is the largest of the secondary lymphoid organs (Figure 2.4A). It is highly efficient in trapping and concentrating foreign substances carried in the blood. It is the major organ in the body in which antibodies are synthesized and from which they are released into the circulation. The spleen is composed of white pulp, rich in lymphoid cells, and red pulp, which contains many sinuses as well as large quantities of erythrocytes and macrophages, some lymphocytes, and a few other cell types. Figure 2.4B shows a section of human spleen illustrating the red and white pulp areas.
The areas of white pulp are located mainly around small arterioles, the peripheral regions of which are rich in T cells; B cells are present mainly in germinal centers. Approximately 50% of spleen cells are B lymphocytes; 30–40% are T lymphocytes. After antigenic stimulation, the germinal centers contain large numbers of B cells and plasma cells. These cells synthesize and release antibodies.
Lymph Nodes
Lymph nodes are small ovoid structures (normally less than 1 cm in diameter) found in various regions throughout the body (see Figure 2.2). The human body contains hundreds of lymph nodes located deep inside the body. They are close to major junctions of the lymphatic channels, which are connected to the thoracic duct. The thoracic duct transports lymph and lymphocytes to the vena cava, the vessel that carries blood to the right side of the heart (see Figure 2.14), from where it is redistributed throughout the body.
Lymph nodes are composed of a medulla, with many sinuses, and a cortex, which is surrounded by a capsule of connective tissue (Figure 2.5A). The cortical region contains primary lymphoid follicles. After antigenic stimulation, these structures enlarge to form secondary lymphoid follicles with germinal centers containing dense populations of lymphocytes (mostly B cells) that are undergoing mitosis (Figure 2.5B). In response to antigen stimulation, antigen‐specific B cells proliferating within these germinal centers also undergo a process known as affinity maturation to generate clones of cells with higher affinity receptors (antibody) for the antigenic epitope that triggered the initial response (see Chapter 9). The remaining antigen‐nonspecific B cells are pushed to the outside to form the mantle zone. The deep cortical area or paracortical region contains T cells and dendritic cells. Antigens are brought into these areas by dendritic cells, which present antigen fragments (peptides) to T cells, events that result in activation of the T cells. The medullary area of the lymph node contains antibody‐secreting plasma cells that have traveled from the cortex to the medulla via lymphatic vessels.
MAJOR HEMATOPOIETIC CELLS
In Chapter 1, we illustrated the developmental pathways of the major hematopoietic cells derived from stem cells in the bone marrow (see Figure 1.1). Cell populations emerging from these pathways include the granulocytes, lymphocytes, erythrocytes, monocytes, macrophages, dendritic cells, and megakaryocytes Our focus here will be those cells responsible for adaptive and innate immune responses which mainly derive from the common lymphoid progenitor cells and myeloid progenitor cells, respectively.
Lymphoid Lineage Cell Populations
We begin our discussion of cells of the immune system by focusing on cells derived from common lymphoid progenitors (CLPs) (see Figure 1.1). These cells, broadly called lymphocytes, represent the principal cell players in adaptive immune responses while, in the case of some cell types, they have the ability to bridge innate and adaptive immunity. Figure 2.6 illustrates the major lymphoid populations of cells derived from CLPs.
B Lymphocytes.
Antibody‐producing
