Lymphatic system

The lymphatic system forms an extensive network throughout the body and is composed of capillaries, collecting vessels, lymph nodes, and lymphoid organs. The bone marrow and thymus, which are the central lymphoid organs, provide the center for the production and maturation of the immune cells.¹ The peripheral lymphoid organs—the spleen, tonsils, appendix, and lymphatic tissue in the respiratory, gastrointestinal, and reproductive systems—concentrate antigens or immunogens and promote the cellular interactions of the immune response throughout the body to seek out and destroy microbes.¹

Lymph is a clear, colorless fluid filtered and collected from the organs and tissues through the lymphatic capillaries. The fluid consists of white bloods cells and occasionally red blood cells. The collecting vessels carry the lymph from the lymphatic capillaries to the bloodstream. The lymph is deposited into the bloodstream through the jugular and subclavian veins in the neck. The lymphatic system also absorbs fat and fat-soluble substances from the intestinal wall. The lymph and fat are transported from the lymph glands to the larger ducts and through the venous return to the heart. The lymphatic system plays a major role in the maintenance of fluid balance, and filters fluid at the lymph nodes and removes bacteria. Obstruction of lymph flow or removal of lymph nodes causes lymphedema.

Lymph nodes are small aggregates of lymphoid tissue lying along lymphatic vessels throughout the body and consist of outer cortical layers and an inner medullary layer. The terms lymph gland and lymph node are often used interchangeably, and both terms can be applied to the lymphatic system. A gland is an organ that produces a substance or secretion, and a node is a swelling or protuberance. The lymph nodes throughout the body are filters for the collection vessels. Each lymph node processes lymph from the surrounding anatomical area. Lymph nodes remove antigens and microbes from the lymph before it enters the bloodstream, serve as the site of the body’s immune response, and aid in the maturation of lymphocytes and monocytes. The T lymphocytes are responsible for cell-mediated immunity and aid in antibody production. They are activated in the cortex of the lymph nodes and proliferate to fight antigens. The B lymphocytes are essential for humoral immunity. They interact with the T lymphocytes and migrate to the medulla to mature before releasing antibodies. Many interactions in the immune system depend upon the secretion of chemical mediators such as cytokines and chemokines. Cytokines are soluble proteins secreted by cells of the immune system and mediate many functions within the cell. Chemokines are cytokines that stimulate the immune system and activate inflammatory cells.¹ They are implicated in acute and chronic conditions such as inflammatory bowel disease, asthma, and rheumatoid arthritis. Figure 10-1 illustrates the lymph glands in the head and neck area and gives a view of the lymphatic chain in the body.

The tonsils and adenoids are organs of the lymphatic system. The buds of tonsillar tissue are present in the oropharynx at birth, but are underdeveloped. As the immune system develops and reacts to respiratory triggers, such as viral, bacterial, and fungal infections and environmental toxins, the tonsils are the first line of defense. The tonsils harbor the immune cells needed to respond to the constant exposure of microorganisms. The adenoidal tissue is a mass of lymphoid tissue situated posterior to the nasal cavity. The adenoids are also known as a pharyngeal tonsil or nasopharyngeal tonsil. Significant hypertrophy of the adenoidal tissue can partially or completely block airflow through the nasal passages and impact the voice.

The thymus gland is embedded beneath the upper sternum above the heart and is a fully developed organ in the term infant. The thymus gland is prominent in the mediastinum of the newborn and infant in the first year of life, often shadowing the cardiac silhouette on radiographs (Figure 10-2). In the infant, the thymus begins to produce mature T lymphocytes and plays an important role in cell-mediated immunity. In puberty, when the immune system is well established, the thymus decreases in size and is gradually and almost entirely replaced by adipose tissue. Some thymus tissue persists, but is usually undetectable in the adult.

The spleen lies in the upper left quadrant of the abdomen protected by the rib cage. It is composed of lymphoid tissue and reticuloendothelial cells and is a densely vascular organ. The spleen acts to filter antigens in the bloodstream and responds to systemic infections. The spleen acts as a part of the immune system and hosts a sequence of activation events similar to the lymph nodes to fight blood-borne infection. In the infant and young child, the spleen stores erythrocytes and filters the blood through the large presence of phagocytes. The spleen plays an important role in the immune system and the storage of erythrocytes, but it is a nonvital organ in the body.