The lymphatic system consists of a fluid (lymph), vessels that transport the lymph, and organs that contain lymphoid tissue. It is often called the secondary circulatory system. The lymphatic system has three primary functions:
• The removal of excess intresticial fluids from body tissues.
• The absorption of fats from the small intestine and transport of those fats to the cardiovascular system.
• The third and probably most well known function of the lymphatic system is defense against invading microorganisms and disease. Lymph nodes and other lymphatic organs filter the lymph to remove microorganisms and other foreign particles.
Lymph originates as blood plasma that leaks from the capillaries of the cardiovascular system. This blood plasma fills the space between individual
cells of tissue where it becomes part of the interstitial fluid. Most of the interstitial fluid is returned to the capillaries. The excess interstitial fluid is collected by the lymphatic system into lymph capillaries, and is processed by lymph nodes before to being returned to the circulatory system. Once within the lymphatic system the fluid is called lymph, and has almost the same composition as the original interstitial fluid. Returning the fluid to the blood prevents edema and helps to maintain normal blood volume and pressure.
Lymphatic vessels, unlike blood vessels, only carry fluid away from the tissues. The smallest lymphatic vessels are the lymph capillaries, which begin in the tissue spaces as blind-ended sacs. There isn’t a central pump, lymph movement occurs slowly with low pressure due to the squeezing action of skeletal muscles. Lymph travels through lymp vessels that are similar to capillaries and veins. Lymph moves in one direction only, due to valves in lymph vessels that are similar to the valves found in veins, shown in Figure above. The lymph is transported to progressively larger lymphatic vessels that drain into the circulatory system at the right and left subclavian veins.
Lymphatic organs are characterized by clusters of leukocyte. When the body is exposed to microorganisms and other foreign substances, the leukocytes proliferate within the lymphatic organs and are sent in the blood to the site of the invasion. This is part of the immune response that attempts to destroy the invading agent.
The four types of lymphatic organs :
Lymph nodes are small bean-shaped structures that are usually less than 2.5 cm in length. They are widely distributed throughout the body along the lymphatic vessels where they filter the lymph before it is returned to the blood.
Tonsils are clusters of lymphatic tissue just under the mucous membranes that line the nose, mouth, and throat (pharynx).
The spleen is located in the upper left abdominal cavity, just beneath the diaphragm, and posterior to the stomach. The spleen filters blood in much the way that the lymph nodes filter lymph. The spleen, along with the liver, removes old and damaged erythrocytes from the circulating blood. Like other lymphatic tissue, it produces leukoocytes, especially in response to invading pathogens
The thymus is located anterior to the ascending aorta and posterior to the sternum. It is relatively large in infants and children but after puberty it begins to decrease in size so that in older adults it is quite small. Thymus is where the lymphocytes are processed.
In this link you can complete the description of the lymphatic organs.