Anatomy and Physiology

• Diffuse, unencapsulated bundles of lymphatic cells. This kind of lymphatic tissue consists of lymphocytes and macrophages associated with a reticular fiber network. It occurs in the lamina propria (middle layer) of the mucus membranes (mucosae) that line the respiratory and gastrointestinal tracts.

• Discrete, unencapsulated bundles of lymphatic cells, called lymphatic nodules (follicles). These bundles have clear boundaries that separate them from neighboring cells. Nodules occur within the lamina propria of the mucus membranes that line the gastrointestinal, respiratory, reproductive, and urinary tracts. They are referred to as mucosa‐associated lymphoid tissue (MALT). The nodules contain lymphocytes and macrophages that protect against bacteria and other pathogens that may enter these passages with food, air, or urine. Nodules occur as solitary nodules, or they cluster as patches or aggregates. Here are the major clusters of nodules:

  • Peyer's patches are clusters of lymphatic nodules that occur in the mucosa that lines the ileum of the small intestine.

  • The tonsils are aggregates of lymphatic nodules that occur in the mucosa that lines the pharynx (throat). Each of the seven tonsils that form a ring around the pharynx are named for their specific region: a single pharyngeal tonsil ( adenoid) in the rear wall of the nasopharynx, two palatine tonsils on each side wall of the oral cavity at its entrance in the throat, two lingual tonsils at the base of the tongue, and two small tubal tonsils in the pharynx at the entrance to the auditory tubes.

  • The appendix, a small fingerlike attachment to the beginning of the large intestine, is lined with aggregates of lymph nodules.

• Encapsulated organs contain lymphatic nodules and diffuse lymphatic cells surrounded by a capsule of dense connective tissue. The three lymphatic organs are discussed in the following sections.

Lymph nodes

Lymph nodes are small, oval, or bean‐shaped bodies that occur along lymphatic vessels. They are abundant where lymphatic vessels merge to form trunks, especially in the inguinal (groin), axillary (armpit), and mammary gland areas. Lymph flows into a node through afferent lymphatic vessels that enter the convex side of a node. It exits the node at the hilus, the indented region on the opposite, concave side of the node, through efferent lymphatic vessels. Efferent vessels contain valves that restrict lymph to movement in one direction out of the lymph node. The number of efferent vessels leaving the lymph node is fewer than the number of afferent vessels entering, slowing the flow of lymph through the node.

Lymph nodes perform three functions:

• They filter the lymph, preventing the spread of microorganisms and toxins that enter interstitial fluids.

• They destroy bacteria, toxins, and particulate matter through the phagocytic action of macrophages.

• They produce antibodies through the activity of B cells.

The structure of a lymph node is characterized by the following features:

• There is a capsule of dense connective tissue that surrounds the lymph node.

• Trabeculae are projections of the capsule that extend into the node, forming compartments. The trabeculae support reticular fibers that form a network that supports lymphocytes.

• The cortex is the dense, outer region of the node. It contains lymphatic nodules where B cells and macrophages proliferate.

• The medulla is the center of the node. Less dense than the surrounding cortex, the medulla primarily contains T cells.

• Medullary cords are strands of reticular fibers with lymphocytes and macrophages that extend from the cortex toward the hilus.

• Sinuses are passageways through the cortex and medulla through which lymph moves toward the hilus.

Thymus

The thymus is a bilobed organ located in the upper chest region between the lungs, posterior to the sternum. It grows during childhood and reaches its maximum size of 40 g at puberty. It then slowly decreases in size as it is replaced by adipose and areolar connective tissue. By age 65, it weighs about 6 g.

Each lobe of the thymus is surrounded by a capsule of connective tissue. Lobules produced by trabeculae (inward extensions of the capsule) are characterized by an outer cortex and inner medulla. The following cells are present:

• Lymphocytes consist almost entirely of T cells.

• Epithelial‐reticular cells resemble reticular cells, but do not form reticular fibers. Instead, these star‐shaped cells form a reticular network by interlocking their slender cellular processes (extensions). These processes are held together by desmosomes, cell junctions formed by protein fibers. Epithelial‐reticular cells produce thymosin and other hormones believed to promote the maturation of T cells.

The function of the thymus is to promote the maturation of T lymphocytes. Immature T cells migrate through the blood from the red bone marrow to the thymus. Within the thymus, the immature T cells concentrate in the cortex, where they continue their development. Mature T cells leave the thymus by way of blood vessels or efferent lymphatic vessels, migrating to other lymphatic tissues and organs where they become active (immunocompetent) in immune responses. The thymus does not provide a filtering function similar to lymph nodes (there are no afferent lymphatic vessels leading into the thymus), and unlike all other centers of lymphatic tissues, the thymus does not play a direct role in immune responses.

Blood vessels that permeate the thymus are surrounded by epithelial‐reticular cells. These cells establish a protective blood‐thymus barrier that prevents the entrance of antigens from the blood and into the thymus where T cells are maturing. Thus, an antigen‐free environment is maintained for the development of T cells.

Spleen

Measuring about 12 cm (5 inches) in length, the spleen is the largest lymphatic organ. It is located on the left side of the body, inferior to the diaphragm and at the left edge of the stomach. Like other lymphatic organs, the spleen is surrounded by a capsule whose extensions into the spleen form trabeculae. The splenic artery, splenic vein, nerves, and efferent lymphatic vessels pass through the hilus of the spleen located on its slightly concave, upper surface. There are two distinct areas within the spleen:

• White pulp consists of reticular fibers and lymphocytes in nodules that resemble the nodules of lymph nodes.

• Red pulp consists of venous sinuses filled with blood. Splenic cords consisting of reticular connective tissue, macrophages, and lymphocytes form a mesh between the venous sinuses and act as a filter as blood passes between arterial vessels and the sinuses.

The functions of the spleen include the following:

• The spleen filters the blood. Macrophages in the spleen remove bacteria and other pathogens, cellular debris, and aged blood cells. There are no afferent lymphatic vessels, and unlike lymph nodes, the spleen does not filter lymph.

• The spleen destroys old red blood cells and recycles their parts. It removes the iron from heme groups and binds the iron to the storage protein.

• The spleen provides a reservoir of blood. The diffuse nature of the red pulp retains large quantities of blood, which can be directed to the circulation when necessary. One third of the blood platelets are stored in the spleen.

• The spleen is active in immune responses. T cells proliferate in the white pulp before returning to the blood to attack nonself cells when necessary. B cells proliferate in the white pulp, producing plasma cells and antibodies that return to the blood to inactivate antigens.

• The spleen produces blood cells. Red and white blood cells are produced in the spleen during fetal development.