Lymphatic vessels are found in almost every tissue of the body that has blood vessels. The smallest lymphatic vessels are referred to as lymph capillaries. They are composed of simple squamous epithelial cells that slightly overlap each other. The gaps between neighboring cells allow the movement of excess tissue fluid into the lymphatic capillary. Once the fluid is picked up by the lymph capillary, it is called lymph. Lymphatic capillaries converge to form lymphatic vessels. These vessels carry the lymph through the lymph nodes to the lymphatic trunk and thoracic duct and then to the left and right subclavian veins where the lymph is dumped into the bloodstream. (You may click on this link to see an image depicting the dumping of lymph fluid back to the large vessels above the hear: lymph dumps into blood stream). The lymph from the upper right side of the body drains into the right subclavian vein via the right lymphatic duct. The lymph from the rest of the body drains into the left subclavian vein via the thoracic duct, which is the largest lymphatic vessel. While blood in the cardiovascular system is circulated due to the pumping of the heart, the lymphatic system depends on contraction of the lymph vessels themselves, as well as muscle pumps and the movements of the body to return the lymph to the heart. As the body moves, the contraction of the skeletal muscles knead and squeeze the lymph vessels pushing the lymph upward. Additionally, the act of breathing and subsequent changes in pressure in the thoracic cavity contributes to drawing the lymph toward the heart. Lymph vessels contain one-way semilunar valves, similar to veins, that prevent lymph from flowing backwards and keep it moving forward until it reaches the heart.

Lymph Nodes

Lymphatic vessels are connected to many lymph nodes. As the lymph is moving toward the bloodstream through the lymphatic vessels, it is filtered through the lymph nodes, removing debris and pathogens. Lymph nodes contain many white blood cells that help defend our bodies from pathogens.

The three main types of white blood cells found in the lymph nodes are macrophages, B-cells, and T-cells.

Macrophages are specialized cells that can engulf pathogens or other material found in lymph through endocytosis. Once inside the macrophage, enzymes from lysosomes will digest the material. This process of engulfing and digesting particulate matter is called phagocytosis (phago- refers to eating, cyto- means cell, -sis refers to a process; so, phagocytosis is "the process of cell eating"). Once the material has been digested down to simple molecules, it is released from the macrophage through exocytosis and can be used as nutrients for other body cells. B-cells and T-cells can be exposed to pathogens in the lymph nodes and become activated to fight infections. (This process will be discussed in detail later). When the lymph carries larger than normal numbers of pathogens to the lymph nodes, the nodes will become tender and swollen. Therefore, swollen, tender lymph nodes are a sign that there is an infection in the body.

Lymph Nodules

Lymph nodules are accumulations of lymphatic tissue found in close association with the mucous membranes lining the respiratory and digestive tracts. As a result, lymph nodules are commonly referred to as MALT (mucosa-associated lymphoid tissue). The tonsils that surround the opening to the pharynx are large accumulations of lymph nodules. Lymph nodules found around the small intestine are commonly referred to as Peyer’s patches. Lymph nodules are not connected to the lymphatic vessels and lymph does not flow through them, but they are similar to lymph nodes in that they house macrophages, B-cells, and T-cells. Many potential pathogens gain access to the body through the respiratory and digestive tracts. Having the lymph nodules right at the mucous membranes (major entry sites into the body) allows these white cells to gain rapid access to potential pathogens that enter these systems. This arrangement allows the white blood cells to mount a rapid defense to destroy the pathogens before they can cause serious harm. Similar to lymph nodes, swollen lymph nodules indicate infection. For example, swollen, tender tonsils would indicate that there is an infection in the pharynx. Frequent and reccurring bacterial or viral infections in the pharynx can lead to enlarged tonsils and chronic tonsillitis, which can make it difficult to swallow or even breath while sleeping. If this condition is severe, an option is to have the tonsils surgically removed in a procedure called a tonsillectomy.

Spleen

The spleen is a soft, spongy organ that has many small blood vessels and sinuses to store blood. In the event of a traumatic injury the spleen can release this stored blood (roughly the volume of a cup) into the circulatory system to replace blood that was lost. Just as the lymph nodes function to remove any pathogens or cellular debris from the lymph, the spleen carries out this function for the blood. As blood flows through the spleen, macrophages can perform phagocytosis on any bacteria or viruses that have gained access to the blood. Thankfully, infections in the blood are fairly uncommon, so the main function of the macrophages in the spleen is to remove dead or dysfunctional red blood cells. Under normal conditions, millions of red blood cells die each second. Many of the components of these red blood cells can be recycled as they are broken down through phagocytosis and dumped back into the blood by the macrophages in the spleen. If pathogens are found in the blood, B-cells and T-cells in the spleen can become activated to respond to the infection.

Thymus

Located between your lungs and behind your sternum, the thymus gland functions as a training and development center for T-lymphocytes (T-cells). Immature T-cells produced in the red bone marrow are sent to the thymus to be trained to only attack foreign invading cells. They begin first in the cortex of the thymus where immature T-cells come in contact with epithelial cells presenting different antigens. Through a process known as “positive selection” the immature T-cells that respond to the different antigens are selected to survive and migrate to the medulla of the thymus, while the remaining T-cells that didn’t’ respond to the different antigens die and are broken down by macrophages. In the medulla, the T-cells are tested for “negative selection” and are presented with the body’s own antigens. If the T-cells attack the body’s own cells (autoimmunity) they are eliminated by apoptosis (self-destruction), resulting in only 2% of immature T-cells reaching maturity. The thymus gland is most active from before birth through puberty as the immune system is actively producing T-cells, and then from puberty on through adulthood the thymus slowly undergoes atrophy (cell death) and is replaced by adipose tissue.

Leukocytes

Leukocytes, or white blood cells, are some of the key players in fighting infection in our bodies. Each leukocyte was given their name based on their appearance when exposed to different pH dyes. Baso = base; basophil granules stain dark blue in basic dye. Eosino = acid; Eosinophil granules stain red or orange in acidic dyes. Neutro = neutral; neutrophils stain a neutral pink in neutral dyes. Lymph = colorless: cyte = cell or vessel; lymphocytes look transparent and will stain pale blue with a dark purple nucleus. Mono = one; monocytes have one large kidney-shaped nucleus inside.

You will be learning more about each of these leukocytes in BIO 265 Lab. Below you will see a chart that briefly describes each of the different leukocytes and their unique characteristics and function.