The lymphatic system is one of the body's natural defences against infection. It is a complex system made up of lymphatic organs, such as bone marrow, tonsils, the spleen, and lymph nodes (also called lymph glands). They are connected by a network of tiny lymphatic vessels. Lymph nodes are mainly found in the neck, armpit and groin. The number of nodes varies from one part of the body to another. In some parts there are very few. However, under your arm there may be between 20 and 50 nodes (see the diagram below).

A milky-looking fluid called lymph circulates through the lymphatic vessels. Lymph contains lymphocytes, which are white blood cells. Lymphocytes are an essential part of the body's defence against infection and disease. For example, if you have a sore throat, you may notice that the lymph nodes in your neck get bigger. This is a sign that your body is fighting the infection.

There are two main types of lymphocyte: B-cells and T-cells. All lymphocytes develop in the bone marrow from immature cells called stem cells. Lymphocytes then mature in different parts of the body. Lymphocytes which mature in the thymus gland (behind the breast bone) are called T-cells. Other lymphocytes mature in the bone marrow or lymphatic organs and are called B-cells.