Lymphatic System and Body Immunity Help
Lymphatic System and Body Immunity
Functioning together with the cardiovascular system, the lymphatic system:
- transports interstitial fluid, called lymph, in lymph vessels from the tissues back to the blood where it contributes to blood plasma;
- assists in fat absorption in the small intestine; and
- plays a key role in protecting the body from bacterial invasion via the blood.
Interstitial fluid enters the lymphatic system through the walls of lymph capillaries, composed of simple squamous epithelium. Lymph is carried then into larger lymphatic ducts. The lymph ducts eventually empty into one of two principal vessels: the right lymphatic duct, which drains lymph from the upper right quadrant of the body, and the large thoracic duct, which drains lymph from the remainder of the body. These drain into the right and left subclavian vein, respectively. Lymph flows through lymphatic vessels through the contraction of skeletal muscle, intestinal peristalsis, and gravity. Valves in lymphatic vessels prevent backflow.
Lymph nodes are small oval bodies enclosed in fibrous capsules. They contain phagocytic cortical tissue adapted to filter lymph. Afferent lymphatic vessels carry lymph into the node and efferent lymphatic vessels carry the filtered lymph from the node. Lymphocytes, leukocytes responsible for body immunity, and macrophages, phagocytic cells, are found in lymph nodes. Lymph nodes occur in clusters or chains. Some principal groups are:
- Popliteal and inguinal nodes of the lower extremity
- Lumbar nodes of the pelvic region
- Cubital and axillary nodes of the upper extremity
- Cervical nodes of the neck
- Mesenteric nodes associated with the small intestine
Lymphoid organs are:
- Tonsils: The three tonsils, (pharyngeal (adenoid), palatine, and lingual), are lymph organs of the pharyngeal region. They function to fight infections of the nose, ear, and throat region.
- Spleen: The spleen is located in the upper left portion of the abdominal cavity. It is not a vital organ in the adult but assists other organs in producing lymphocytes, filtering blood and destroying old erythrocytes. It also serves as a reservoir for erythrocytes.
- Thymus: The thymus is in the anterior thorax, deep to the manubrium of the sternum. It is much larger in a child than an adult. In a child it functions to change undifferentiated lymphocytes into T lymphocytes and is a reservoir for lymphocytes.
Nonspecific Defense Mechanisms
Types of non-specific defense mechanisms include:
- Mechanical barriers: skin and mucous membranes
- Chemical barriers: enzymes, HCl in stomach, lysozymes
- Interferon: proteins that inhibit viral growth
- Phagocytes: neutrophils, monocytes, and macrophages
- Species resistance
Specific immunity refers to resistance of the body to specific foreign agents (antigens). These include microorganisms, viruses, and their toxins, as well as foreign tissue and other substances. Antigens are large complex molecules (proteins, polysaccharides) found on cell walls or membranes of foreign substances.
In antibody-mediated immunity, an antigen stimulates the body to produce special proteins, antibodies, that destroy the particular antigen through an antigen-antibody reaction. Antibodies are gamma globulins composed of four interlinked polypeptide chains, two short (light) chains and two long (heavy) chains (Figure 17-1). All antibodies have constant regions, that are structurally similar, and variable regions, that are the location of antigen-binding sites. Small variations in the variable region make each antibody highly specific for one particular antigen. Antigen-antibody binding induces the production of more antibodies specific for that antigen.
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