Remember that lymphocytes are a key component of adaptive immunity. While all leukocytes originally come from stem cells in the bone marrow, the B lymphocytes (also called B cells) mature in the bone marrow, whereas T lymphocytes (or T cells) mature in the thymus. Together the bone marrow and thymus are the primary lymphoid organs. The secondary lymphoid organs include the lymph nodes, spleen, Peyer’s patches in the small intestine, the appendix, and tonsils. The secondary lymphoid organs are where lymphocytes come into contact with pathogens and are activated. You can think of these secondary lymphoid organs as being like the guard houses and watchtowers along the castle wall. Guards take intruders to the guard house where they are interrogated and the army is called out if necessary. Like the guard stations of a castle, many of these secondary lymphoid organs are also strategically located at sites where invasions are likely. For example, the tonsils guard the nose and mouth from invaders, whereas Peyer’s patches and the appendix guard against invasion from the digestive tract. Secondary lymphoid organs also house macrophages and other immune system cells. The lymph nodes are small oval or bean-shaped secondary lymphoid organs embedded in connective tissue and arrayed along lymphatic vessels. Clusters of lymph nodes are found where several lymphatic vessels converge— for example in the cervical, axillary and inguinal regions. The functions of the lymph nodes are to filter the lymph by removing antigens and other debris that may have entered the lymph, and to enable B and T cells to interact with antigens. These interactions generate immune responses. Here’s a close-up of a lymph node. Lymph nodes are covered by a dense connective tissue capsule. Lymph nodes are separated into sections by bundles of collagen fibers called trabeculae that extend from the capsule deep into the node. Beneath the capsule is the subcapsular sinus. This is the first of a series of sinuses, interconnected dilated channels, through which the lymph flows as it passes through the lymph node. Lymph from the afferent lymphatic vessels empties into the subcapsular sinus and then flows into sinuses in the outer cortex. The outer cortex of the lymph node is the area just below the subcapsular sinus. Here, B cells are found organized into oval-shaped collections of cells called lymphoid follicles. Some of the follicles contain lighter-staining central areas called germinal centers. These are formed by B cells proliferating in response to antigen. Moving inward from the outer cortex, we come to the deep cortex. Lymphocytes exit blood vessels and enter lymph nodes in the deep cortex. T cells congregate in the deep cortex, which is rich with dendritic cells that have captured antigens and are presenting them on their surfaces. T cells wander through the deep cortex searching dendritic cells for that T cell’s special antigen. The central area of the node is the medulla. It is shaped into elongated masses of cells called medullary cords, around which lymph flows. Medullary cords contain both types of lymphocytes as well as macrophages and plasma cells, which are derived from B cells and are antibody-producing factories. Efferent lymphatic vessels leave and blood vessels enter and leave the node at a shallow indentation called the hilum. Let’s observe the flow of lymph through the lymph node. Now let’s look at the spleen. The largest of the lymphoid organs, the spleen is a fist-sized, blood-rich organ located to the left of, and dorsal to, the stomach. The spleen performs the same cleansing function for the blood as the lymph nodes do for the lymph. The spleen removes pathogens and aged erythrocytes and platelets from the blood, stores platelets and breakdown products of red blood cells, and provides a site for the interaction of lymphocytes with antigens. As you explore the structure of the spleen, notice how this structure allows intimate contact between blood and lymphocytes, just as the structure of lymph nodes is designed for intimate contact between lymph and lymphocytes. The mucosal surfaces of the digestive tract, as well as the respiratory tract and genitourinary systems, are vulnerable to invasion by pathogens because they are directly exposed to the external environment. Like guard houses and watchtowers strung along a castle wall, we have collections of lymphoid tissue, called the mucosa-associated lymphoid tissues (MALT), strategically distributed throughout the mucosae. MALT includes the tonsils, appendix, and Peyer’s patches of the small intestine, as well as more diffuse collections of cells in the respiratory tracts and other mucosae. These tissues are unencapsulated or partially encapsulated collections of lymphocytes. MALT contains both B and T cells, with the B cells occurring in lymphoid follicles similar to those found in lymph nodes and the spleen. Peyer’s patches are found in the mucosa of the distal portion of the small intestine. In this photomicrograph you can see the many lymphoid follicles in the small intestine that make up the Peyer’s patches. Like the tonsils and other MALT components, Peyer’s patches are located where they can sample the antigens moving through hollow organs open to the external environment. If a pathogen escapes the defenses of the MALT, it can still be cleared by responses of the lymph nodes or spleen. We have now finished our investigation of MALT, the last of the secondary lymphoid tissues and organs that we will consider. Now, let’s look at the thymus, a primary lymphoid organ. The thymus is the site for differentiation of lymphocytes into mature T cells. Thymic hormones and other factors influence the development of immature T cells. The thymus is a bilobed organ located in the mediastinum. In young children, the thymus is large relative to body size. The relative size of the thymus, as well as its function, gradually decreases with age. In the elderly, thymic epithelial cells are almost entirely replaced by fat cells and fibrous connective tissue. This process, called thymic atrophy, may be one reason why the elderly are more susceptible to infection. Let’s observe thymic atrophy. Now we’ll view the organ more closely. Each lobe of the thymus is divided into many lobules. Each lobule contains an outer cortex… and an inner medulla. Most of the cells in the thymus are immature T cells at various stages of development. Scattered amongst the T cells are thymic epithelial cells which influence T cell development and secrete thymic hormones such as thymopoetin and the thymosins. Within the medulla are distinctively shaped structures called thymic corpuscles. thymic corpuscles are clusters of keratinized epithelial cells with a whorled appearance that are scattered throughout the medullary area. While their function is not completely understood, they are thought to be involved in the development of a type of T cell called a regulatory T cell.
