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The group of tissues and organs that participate in the immune response is known as lymphatic system. It is made up of organs, vessels, nodes and lymphatic tissue. This system carries out three basic functions:

Defense: in the lymph nodes, the lymphocytes reproduce in order to respond to the antigens.
Fat absorption: most of the fats are absorbed by the lymphatic system and later transported towards the blood.
Capillary exchange: it recovers substances the blood flow has lost in the capillary exchange.

The lymphatic system is made up of two groups of lymphoid organs: primary or central and secondary or peripheral. 
The process known as lymphopoiesis is produced in the primary ones, meaning, the maturation of the lymphocytes in such a way that they acquire a repertoire of specific receptors for each type of antigen.The primary lymphoid organs are the thymus, where the T lymphocytes mature, and bone marrow, where the maturation of the B lymphocytes take place.The function of the secondary ones is to provide the environment for the lymphocytes to interact and come into contact with the antigen, meaning, create a favorable environment for the immune responses to be unleashed. The secondary lymphatic organs are the lymph nodes and tissue, associated to mucosae, and the spleen. 

Lymphatic vessels and nodes

The lymphatic vessels are similar in their structure to blood veins, in fact, every few millimeters a series of valves appear that help the ascension of the lymph. These vessels start out as capillaries that collect the lymphatic liquid and then start to unite in order to form bigger vessels. This way, the lymph is finally collected by two great conduits called thoracic duct and lymphatic duct.

The first one is the most important because it drains over half the body and ends its run in the left subclavian vein. The lymphatic duct eases the exit of the lymph of the right side of the body above the diaphragm and ends in the right subclavian.

The lymph nodes are spread throughout the body, but their presence is most abundant in areas like the axillae, groin, neck and pelvis. They are made up of a resistant layer –of less than one centimeter- with small, round-shaped nodules that share spots with the lymphatic vessels. Inside, there is lymphoid tissue in which you can find numerous lymphocytes and phagocyte cells. The lymph passes through these nodes which have entry and exit canals.In them, there are also germs that are stranded after the passing of lymph, but are promptly destroyed. However, when they are too numerous, the node swells.

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