Defence Against Pathogens
Human Biology (Year 12)
What is the lymphatic system?
The lymphatic system consists of:
A network of lymph vessels and capillaries joined to larger lymph vessels
Lymph nodes, which are located along the length of some lymph vessels
The main function of the lymphatic system is to collect some of the fluid that escapes from the blood capillaries and and return it to the circulatory system. The lymphatic system is also an important part of the bodies internal defence against pathogens.
What are lymph vessels?
Lymph vessels are blind-ended tubes located in the spaces between the cells of most tissues in the body. Lymph vessels are both slightly larger and more permeable than blood capillaries.
Excess fluid located in the tissues is returned to the body via the lymphatic system; fluid returned to the blood in this way is termed lymph. Unlike the circulatory system, lymph does not circulate – lymph is a one way system, which carries fluid away from the tissues. Protein and disease-causing organisms located in the intercellular fluid can easily pass though the walls of the lymph capillaries.
What are lymph nodes?
A lymph node is surrounded by a capsule of connective tissue that extends into the node, forming a framework. Within the tissue are masses of lymphoid tissue, which contain cells known as lymphocytes, macrophages and plasma cells.
Spaces between the cells of the lymphoid tissue are crisscrossed by a network of fibers. Lymph enters through the vessels on the convex side of the node, filters through the spaces and passes out the opposite side. Lymph passes out through several nodes before entering the circulatory system.
Lymph is pushed along lymph vessels by the squeezing action of skeletal muscles (similar to peristalsis), where valves are located to prevent backflow.
What is the role of the lymphatic system in non-specific defence?
Some micro-organisms which make their way into the lymph may be pathogenic; if not destroyed they can cause disease. Larger particles, such as bacteria, are trapped in the mesh work of fibers as the lymph flows through the spaces of the nodes. Macrophages destroy these particles by ingesting them through phagocytosis. When infections occur, the formation of lymphocytes increases and the lymph nodes become swollen and sore. Most lymphocytes are important in the specific immune response to a pathogen.
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