The Lymphatic Drainage System, also called the Lymphatic System is a subsystem of the circulatory system and the body’s first line of defense against infection and diseases. This is made up of a complex network of lymphatic tissues, vessels, and organs that help rid the body of waste and toxins. This bears a striking resemblance to the veins and capillaries of the human circulatory system and are are connected to the lymph nodes.
The lymphatic system transports “lymph”, that is a fluid comprising of infection-fighting white blood cells, throughout the body, helping to maintain balance throughout the human body. These assemble excess fluids and particulate matter from the tissues, finally depositing them into the bloodstream.
The network of lymph vessels is generally valve less and smaller in size and combine to form the pre-collector vessels, that are not fully functional. These structures then combine to create the larger lymphatic vessels which have lymph-angions (lymph hearts or delicate lymph vessels), which act as a wall, preventing backflow of lymph.
The Components of the Lymphatic system are as follows.
- Primary Lymphoid Organs: sites of B and T cell maturation
Thymus
The thymus is a bilobed organ that is located just behind the sternum which consists of an outer, lymphocyte-rich cortex along with an inner medulla. The differentiation or maturity of T cells occurs in the cortex of the thymus. The thymus appears early in fetal development, and continues its growth until the human body attains puberty. It then starts declining with age, leading to decrease in T-cell production.
These developing T cells (also known as thymocytes) start distinguishing between the body’s own components and foreign substances, known as “self” and “non-self” respectively. When exposed to self-molecules of the major histocompatibility complex (MHC), the cells that are capable of recognizing the MHC are preserved, while those failing to do so are destroyed. The thymocytes then move into the medulla of the thymus, where a negative selection occurs, that is, the thymocytes that are prone to attack the body’s own tissues are destroyed.
Bone Marrow
It is the primary organ for development of lymphocytes. Unlike the Thymus, it does not decay at puberty, that is, there is no collateral decrease in the production of lymphocytes with age.
Fetal Liver
A major site of development of T-cells and other aspects of the human immune system. It is also the prenatal site where the B cells undergoes differentiation.
- Secondary Lymphoid Organs: Further Differentiation of Lymphocytes
Lymph Nodes
The Lymph nodes or glands are composed of lymphatic tissues that are mainly prevalent in areas around the neck (cervical nodes), groin (inguinal nodes), armpit (axillary nodes) and knees (popliteal nodes). These nodes contain lymphocytes, which enter the bloodstream through high endothelial venules (specialized vessels).
T-cells are organized in the inner cortex (paracortex) while B cells congregate in the outer cortex (germinal centers). Lymph, combining with the antigens, drains into the node through incoming vessels and filters through the nodes, activating the lymphocytes. Once activated, the lymphocytes exit the node through outgoing vessels, infiltrating the bloodstream and dispensing throughout the body.
The main function of these nodes is production and storage of cells that fight infection and foreign invaders; that is, to filter or decontaminate the lymph before it can be returned to the circulatory system.
Spleen
It is the largest lymphatic organ in the body, containing white blood cells known as macrophages that fight and remove bacteria and infections, located in the abdominal cavity. Although having a structural similarity to the lymph node, it filters blood with its functional tissue made up of:
- Red pulp, containing macrophages.
- White pulp (in the surrounding regions) containing both T and B lymphocytes
Mucosa-associated tissues
They are generally associated with the mucosal surface of almost any organs, especially those present in the digestive, genitourinary and respiratory tracts. These are constantly exposed and require their very own system of antigen detention and presentation to the lymphocytes.
Tonsils
These are essentially a couple of lymph nodes on both sides of the throat and whose primary function is to be the first line of defense against bacteria and viruses that enter through the mouth. They also produce WBCs.
Functions:
Lymphocyte functions are summed up as:
- Mature B lymphocytes oversee the production of antibodies, which are tiny protein molecules that stick against the pathogens, to be identified for the macrophage or T lymphocytes to destroy them.
- T lymphocytes act as memory cells, activating the immunity system against certain pathogens.
The primary functions of the lymphatic system are:
- Maintaining fluid balance by removing excess fluid and other waste products within the interstitial spaces between the cells and also the transportation of lymph fluid to lymph nodes.
- Fat absorption, from the digestive system and delivering these nutrients to different cells of the body for further usage.
- Immunological defense against pathogens or harmful undesirables such as waste materials, debris, cancer cells, and toxins.
The lymphatic system along with the lymphocytes present abundantly throughout the body (in trillions), form a part of what immunologists refer to as the "adaptive immune response" - highly specific and long-lasting responses to specific pathogens.