The lymphatic system

Lymph is a clear fluid that contains lymphocytes, a type of white blood that fights germs, foreign substances and abnormal cells, like cancer cells. Lymph also has macrophages, another type of white blood cell that helps fight infection.

Lymph collects some waste products, bacteria and abnormal cells from inside the body's tissues so that they can be removed from the body or destroyed. Lymph drains into lymph vessels that carry it to lymph nodes. Lymph nodes clean the lymph and add more lymphocytes to it.

Lymphocytes fight disease and micro-organisms that cause infections, like bacteria, viruses, fungi and parasites. They are important cells in the body’s immune system.

There are 3 types of lymphocytes:

• B cells (also called B lymphocytes) make antibodies to fight an infection.
• T cells (also called T lymphocytes) defend the body against disease and infection and controls how the body responds to them.
• Natural killer cells attack abnormal cells. These may include cells infected with a virus or cancer cells.

Lymph vessels are tubes that carry lymph through the body to lymph nodes and back to veins. The network of lymph vessels is similar to blood vessels (arteries and veins) that carry blood. Lymph vessels carry waste products, germs and abnormal cells away from the body’s tissues.

The thoracic duct is the largest lymph vessel in the body. It carries lymph from the chest to the abdomen.

Lymph nodes are small, bean-shaped masses of lymphatic tissue along lymph vessels. They store lymphocytes and filter waste, bacteria and abnormal cells (including cancer cells) from lymph. The lymphocytes inside the lymph nodes also attack bacteria and viruses that they find in the lymph. This is why lymph nodes often get swollen when we are sick or fighting off an illness like a cold or the flu.

Lymph nodes are found in many parts of the body. The number of lymph nodes varies from one part of the body to another. Lymph nodes are located in groups, mainly in the:

• neck (called cervical lymph nodes)
• chest (called thoracic and mediastinal lymph nodes)
• armpit (called axillary nodes)
• abdomen (called para-aortic, peri-aortic and mesenteric lymph nodes)
• groin (called inguinal lymph nodes)

Tonsils are small masses of tissue at the back of the mouth and nose and at the top of the throat. Tonsils have many lymphocytes.

The adenoid (pharyngeal tonsil) is a single, small mass of lymphatic tissue in the back of the nose that contains lymphocytes. The adenoid is largest in children and starts to shrink just before puberty. Although it is often called “the adenoids,” there is only 1 adenoid.

The tonsils and adenoid help the body fight infection and prevent bacteria and viruses from entering the digestive system and lungs.

The spleen is an organ in the abdomen, under the ribs on the left side of the body. It stores lymphocytes, filters the blood and destroys old blood cells. As blood passes through the spleen, lymphocytes attack any bacteria or virus or other harmful substances that can cause infection or illness.

The thymus is a gland in the middle of the chest behind the sternum (breastbone). It sits in an area of the chest called the mediastinum. The thymus starts to shrink during late childhood and adolescence, becoming very small in adults.

T cells mature (develop) inside the thymus. Once they are mature, the T cells travel to the lymph nodes through the blood stream. Once in the lymph nodes, they help the immune system fight disease and infection.

The lymphatic system also includes areas of the body that have large amounts of lymphocytes. These areas are called lymphatic tissue. They include the appendix and areas of the small intestine called Peyer’s patches. Some lymphatic tissue can also be found throughout other parts of the digestive system and respiratory system.

Like lymph nodes, lymphatic tissue helps remove waste, bacteria and abnormal cells from lymph.

The bone marrow is the soft, spongy area inside of most bones. It is where lymphocytes and other types of blood cells are made.

Many of the cells in the bone marrow have not fully developed, or matured. These immature cells are called stem cells. Stem cells change and grow into different types of cells, including blood cells. Most blood cells grow and mature in the bone marrow. They then leave the bone marrow and move into the blood and other areas of the body.