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 thymus, the spleen, and lymph nodes. They are connected by a network of tiny lymphatic vessels. Lymph nodes are found mainly in the neck, armpit and groin. Their number varies from one part of the body to another. In some parts of the body there are very few, whereas, for example, under your arm there may be between 20 and 50 nodes (see diagram below).

Circulating through the lymphatic vessels is milky-looking fluid called lymph, which contains lymphocytes. Lymphocytes are white blood cells. They 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 larger. 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 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.