Lymph is the name for tissue fluid that enters lymph capillaries. Filtration in capillaries creates tissue fluid from blood plasma, most of which returns almost immediately to the blood in the capillaries by osmosis. Some tissue fluid, however, remains in interstitial spaces and must be returned to the blood by way of the lymphatic vessels. Without this return, blood volume and blood pressure would very soon decrease. The relationship of the lymphatic vessels to the cardiovascular system is depicted.
The system of lymph vessels begins as dead-end lymph capillaries found in most tissue spaces. Lymph capillaries are very permeable and collect tissue fluid and proteins. Lacteals are specialized lymph capillaries in the villi of the small intestine; they absorb the fat-soluble end products of digestion, such as fatty acids and vitamins A, D, E, and K. Lymph capillaries unite to form larger lymph vessels, whose structure is very much like that of veins. There is no pump for lymph (as the heart is the pump for blood), but the lymph is kept moving within lymph vessels by the same mechanisms that promote venous return. The smooth muscle layer of the larger lymph vessels constricts, and the one-way valves (just like those of veins) prevent backflow of lymph.
Lymphatic tissue consists mainly of lymphocytes in a mesh-like framework of connective tissue. Recall that most lymphocytes are produced from stem cells in the red bone marrow, then migrate to the lymph nodes and nodules, to the spleen, and to the thymus. In these structures, lymphocytes become activated and proliferate in response to infection (this is a function of all lymphatic tissue). The thymus has stem cells that produce a significant portion of the T lymphocytes
LYMPH NODES AND NODULES
Lymph nodes and nodules are masses of lymphatic tissue. Nodes and nodules differ with respect to size and location. Nodes are usually larger, 10 to 20 mm in length, and are encapsulated; nodules range from a fraction of a millimeter to several millimeters in length and do not have capsules. Lymph nodes are found in groups along the pathways of lymph vessels, and lymph flows through these nodes on its way to the subclavian veins.
The spleen is located in the upper left quadrant of the abdominal cavity, just below the diaphragm, behind the stomach. The lower rib cage protects the spleen from physical trauma. In the fetus, the spleen produces red blood cells, a function assumed by the red bone marrow after birth. After birth the spleen is very much like a large lymph node, except that its functions affect the blood that flows through it rather than lymph.
Innate immunity has several aspects: anatomic and physiological barriers, phagocytic and other defensive cells, and chemical secretions and reactions, including inflammation. These are not separate and distinct; rather there is a great deal of overlap among them, as you will see. The innate immune responses are always the same, and their degree of efficiency does not increase with repeated exposure.
The stratum conium of the epidermis of the skin is non-living, and when unbroken is an excellent barrier to pathogens of all kinds. The fatty acids in sebum help limit the growth of bacteria on the skin. The living cells of the epidermis produce defensing, which are antimicrobial chemicals. The mucous membranes of the respiratory, digestive, urinary, and reproductive tracts are living tissue, yet still a good barrier.
Macrophages, both fixed and wandering, have receptors for the pathogens humans are likely to encounter (this probably reflects millions of years of coexistence) and are very efficient phagocytes. Other cells capable of phagocytosis of pathogens or other foreign antigens are the neutrophils and, to a lesser extent, the eosinophil’s. Phagocytic cells use intracellular enzymes and chemicals such as hydrogen peroxide (H2O2) to destroy ingested pathogens.
Chemicals that help the body resist infection include the interferon’s, complement, and the chemicals involved in inflammation. The interferon’s (alpha-, beta-, and gamma-interferon’s) are proteins produced by cells infected with viruses and by T cells. Viruses must be inside a living cell to reproduce, and although interferon cannot prevent the entry of viruses into cells, it does block their reproduction. When viral reproduction is blocked, the viruses cannot infect new cells and cause disease.
The preceding discussions of immunity will give you a small idea of the complexity of the body’s defense system. However, there is still much more to be learned, especially about the effects of the nervous system and endocrine system on immunity. For example, it is known that people under great stress have immune systems that may not function as they did when stress was absent. At present, much research is being done in this field. The goal is not to eliminate all disease, for that would not be possible. Rather, the aim is to enable people to live healthier lives by preventing certain diseases.