The lymphatic system is part of the body's immune system. It plays a key role in the body's defences against infection and some other types of disease, including cancer.
Like the blood system, the lymphatic system is circulatory, but has a fluid known as lymph flowing through it, rather than blood. The lymphatic system helps to transport substances - cells, proteins, nutrients, waste products - around the body. It includes lymphatic vessels (sometimes called simply 'lymphatics'), lymph nodes (sometimes called 'lymph glands') and organs such as the spleen and thymus.
Physiology and role of the lymphatic system
The lymphatic system is an important part of the body's immune system, providing defence against infection and some other types of disease, including cancer.
A fluid called lymph circulates through the lymphatic vessels, and carries lymphocytes (white blood cells) around the body.
The lymphatic vessels pass through the lymph nodes. The lymph nodes contain large numbers of lymphocytes and act like filters, trapping infecting organisms such as bacteria and viruses.
Lymph nodes tend to cluster together in groups - for example, there are large groups in the armpits, in the neck and in the groins.
When a part of the body is infected or inflamed, the nearest lymph nodes often become enlarged and tender. This is what happens, for example, when a person with a sore throat develops 'swollen glands' in the neck. The lymphatic fluid from the throat drains into the lymph nodes in the neck, where the infecting organism can be destroyed and prevented from spreading to other parts of the body.
Importance of T and B cells
There are two main types of lymphocytes:
Lymphocytes, just like other types of blood cells, develop in the bone marrow. They start life as immature cells called stem cells. In early childhood, some lymphocytes then migrate to the thymus, an organ in the top of the chest, where they mature to become T cells. Others remain in the bone marrow and mature there to become B cells. Both T cells and B cells play an important role in recognising and destroying infecting organisms such as bacteria and viruses.
In normal conditions, most of the lymphocytes circulating in the body are T cells. Their role is to recognise and destroy abnormal body cells (for example, cells that have been infected by a virus).
B cells recognise 'foreign' cells and material (for example, bacteria that have invaded the body). When these cells come into contact with a foreign protein (for example, on the surface of bacteria), they produce antibodies, which then 'stick' to the surface of the foreign cell and cause its destruction.