The Lymphoid System

Raylene S Armour
Anatomy 1- Monday Lab
Extra Credit

The lymphoid system provides the central structure of the immune system for the body. It is similar to the circulatory system in that it runs throughout the entire body; these two systems work closely with each other to achieve optimum results. The lymphoid system is a complex network composed of lymph, lymphoid vessels, tissues, and organs. These elements work in tandem to carry fluid from the tissues back into the bloodstream. The fluid that runs inside is a clear, colorless liquid called lymph. It consists of interstitial fluid, proteins, fats, hormones, bacteria, viruses, cellular debris, and macrophages. Lymph also contains lymphocytes, which are the cells of the lymphoid system. The lymph is transported by the lymphoid vessels, and passes through the lymphoid tissues and organs before making its way back to the heart. Aside from providing immune support, there are three primary functions of the lymphoid system. First, to produce, maintain, and distribute lymphocytes. The bone marrow and thymus are the primary lymphoid structures where lymphocytes develop and mature. The secondary structures are the lymph nodes and tonsils where immune responses take place. The second function is to maintain normal blood volume and regulate the composition of the interstitial fluid. The lymphoid vessels move an equal volume of interstitial fluid from the blood to the tissues and back to the blood. This daily circulation aids in keeping a constant balance of fluid, as well as its contents. Third, the lymphoid system provides an alternative transport route for hormones, nutrients, and waste products. This is useful for example for lipids, which when absorbed by the digestive tract are too big to pass across the capillary walls. So the will be carried to the blood by the lymphatic system instead. The lymphatic vessels operate on a closed circuit, carrying lymph only from peripheral tissues to the veins. The small diameter vessels are the lymphatic capillaries, and the large diameter vessels are the lymphatic ducts. The flow of lymph begins at the capillaries, as it is collected from the interstitial fluid found surrounding the tissue cells of the body. From the capillaries, lymph flows into larger lymphatic vessels that contain internal valves; these valves prevent backflow of lymph within the vesicles. Once fluid is in the lymph system, it flows through the nodes and lymph organs, where it is filtered and cleaned. There are two major lymph collectors in the body, the thoracic duct and the right lymphatic duct. The thoracic duct collects most of the lymph in the body from the left side head, neck, thorax, and the entire body below the diaphragm. The small right lymphatic duct only collects from the right side of the body above the diaphragm. Lymphocytes are in charge of guarding the body’s immune response by physically and chemically attacking bacteria, viruses, abnormal cells, and foreign proteins or toxins. They circulate the entire body, through the bloodstream to peripheral tissues and back into the blood. There are three types of lymphocytes in the blood: T cells, B cells, and natural killer cells. The body has two ways in which it initiates the immune response, either a direct attack by active T-cells, or an attack by antibodies released from plasmocytes of B cells. When an antigen binds to a lymphocyte, it activates, dividing to produce more cells specific to the antigen. Some lymphocytes will kill the antigen immediately and others will be ready in case the antigen enters the body another time. This allows for immediate and long term defense against the antigen. Connective tissue that contains an abundance of lymphocytes are lymphoid tissues. An example of this is the tonsils. They are large nodules in the