Organs, Tissues, and Cells of the Immune System
The immune system, which plays an important role in keeping us healthy, consists of a network of lymphatic organs, tissues, and cells as well as products of these cells, including antibodies and regulatory agents. Immunity is the ability to react to antigens so that the body remains free of disease. Disease, a state of homeostatic imbalance, can be due to infection and/or to the failure of the immune system to function properly.
Primary Lymphatic Organs
Lymphatic (lymphoid) organs contain large numbers of lymphocytes, the type of white blood cell that plays a pivotal role in immunity. The primary lymphatic organs are the red bone marrow and the thymus gland (Fig. 13.2, left). Lymphocytes originate and/or mature in these organs.
Red Bone Marrow
Red bone marrow is the site of stem cells that are ever capable of dividing and producing blood cells. Some of these cells become the various types of white blood cells: neutrophils, eosinophils, basophils, lymphocytes, and monocytes (Fig. 13.3).
In a child, most bones have red bone marrow, but in an adult it is limited to the sternum, vertebrae, ribs, part of the pelvic girdle, and the proximal heads of the humerus and femur.
The red bone marrow consists of a network of reticular tissue fibers, which support the stem cells and their progeny. They are packed around thin-walled sinuses filled with venous blood. Differentiated blood cells enter the bloodstream at these sinuses.
Lymphocytes differentiate into the B lymphocytes and the T lymphocytes. Bone marrow is not only the source of B lymphocytes, but also the place where B lymphocytes mature. T lymphocytes mature in the thymus.
Figure 13.2 The lymphatic organs. Left: The red bone marrow and thymus gland are the primary lymphatic organs. Right: Lymph nodes and the spleen, as well as other lymphatic organs such as the tonsils, are secondary lymphatic organs.
Figure 13.3 The five types of white blood cells. These cell types differ according to structure and function. The frequency of each type of cell is given as a percentage of the total.
The soft, bilobed thymus gland is located in the thoracic cavity between the trachea and the sternum superior to the heart. The thymus varies in size, but it is largest in children and shrinks as we get older. Connective tissue divides the thymus into lobules, which are filled with lymphocytes. The thymus gland produces thymic hormones, such as thymosin, that are thought to aid in the maturation of T lymphocytes. Thymosin may also have other functions in immunity.
Immature T lymphocytes migrate from the bone marrow through the bloodstream to the thymus, where they mature. Only about 5% of these cells ever leave the thymus. These T lymphocytes have survived a critical test: If any show the ability to react with “self” cells, they die. If they have potential to attack a foreign cell, they leave the thymus.
The thymus is absolutely critical to immunity; without a thymus, the body does not reject foreign tissues, blood lymphocyte levels are drastically reduced, and the body’s response to most antigens is poor or absent.
Secondary Lymphatic Organs
The secondary lymphatic organs are the spleen, the lymph nodes, and other organs, such as the tonsils, Peyer patches, and the appendix. All the secondary organs are places where lymphocytes encounter and bind with antigens, after which they proliferate and become actively engaged cells.
The spleen, the largest lymphatic organ, is located in the upper left region of the abdominal cavity posterior to the stomach. Connective tissue divides the spleen into partial compartments, each of which contains tissue known as white pulp and red pulp (see Fig. 13.2). The white pulp contains a concentration of lymphocytes; the red pulp, which surrounds venous sinuses, is involved in filtering the blood. Blood entering the spleen must pass through the sinuses before exiting. Lymphocytes and macrophages react to pathogens, and macrophages engulf debris and also remove any old, worn-out red blood cells. The spleen’s outer capsule is relatively thin, and an infection or a blow can cause the spleen to burst. Although the spleen’s functions are replaced by other organs, a person without a spleen is often slightly more susceptible to infections and may have to receive antibiotic therapy indefinitely.
Lymph nodes, which are small, ovoid structures, occur along lymphatic vessels. Connective tissue forms the capsule of a lymph node and also divides the organ into compartments (see Fig. 13.2). Each compartment contains a nodule packed with B lymphocytes and a sinus that increases in size toward the center of the node. As lymph courses through the sinuses, it is filtered by macrophages, which engulf pathogens and debris. T lymphocytes, also present in sinuses, fight infections and attack cancer cells. Each portion of the anterior cavity contains superficial and deep lymph nodes, named for their location. For example, inguinal nodes are in the groin, and axillary nodes are in the armpits. Physicians often feel for the presence of swollen, tender lymph nodes in the neck as evidence that the body is fighting an infection. This is a noninvasive, preliminary way to help make such a diagnosis.
Lymphatic nodules are concentrations of lymphatic tissue not surrounded by a capsule. The tonsils are patches of lymphatic tissue located in a ring about the pharynx. The tonsils perform the same functions as lymph nodes, but because of their location, they are the first to encounter pathogens and antigens that enter the body by way of the nose and mouth.
Peyer patches are located in the intestinal wall, and the appendix. These structures encounter pathogens that enter the body by way of the intestinal tract.