Unit 8 – Introduction to Lymphatic System Tissues

Introduction to Lymphatic System Tissues

The lymphatic system (lymphoid system) is considered a critical component of human immune defense and consists of a variety of immune cells such as lymphocytes and phagocytic cells concentrated in lymphatic organs and an extensive network of lymphatic ducts that drain lymphatic fluid (lymph) from tissues all over the body.  Lymphatic tissues and organs are classified as either primary lymphoid tissue or secondary lymphoid tissue.  Primary lymphoid tissues are locations where new lymphocytes are produced and mature.  The primary lymphoid tissues of the human body are red bone marrow and the thymus.  Secondary lymphoid tissues are organs or structures where lymphocytes are concentrated and actively working to identify and destroy foreign antigens and abnormal cells.  Secondary lymphoid tissues include the spleen, lymph nodes, tonsils, and MALT (mucosa-associated lymphatic tissue).

In addition to immune defense, another important function of the lymphatic system is help maintain normal tissue fluid balance.  Lymphatic capillaries collect a small amount of excess interstitial fluid from tissues all over the body and drain it into a system of larger lymphatic vessels that eventually return the fluid to the circulatory system.  After excess interstitial fluid enters lymphatic capillaries at the peripheral tissue level it is then referred to as lymphatic fluid or simply lymph.  All lymphatic vessels that collect lymph from the body eventually converge into two large vessels known as the right lymphatic duct and the thoracic duct.  The right lymphatic duct collects lymph from all parts of the right side of the body superior to the diaphragm including the right arm and right side of the head, neck, and chest and drains the fluid into the right subclavian vein.  The thoracic duct is significantly larger than the right lymphatic duct and collects lymph from all other regions of the body and drains it into the left subclavian vein.

The tutorials in this unit will focus on the structure and function of the thymus, lymph nodes, and spleen.  MALT (mucosa-associated lymphatic tissue) will be covered under the topic of Peyer’s patches which are clusters of lymphatic follicles in the wall of the ileum of the small intestine.  Peyer’s patches will be covered in the small intestine tutorial chapter of the digestive system unit.