Lymphoma, like other cancers, is a disease of the body's cells. Cells in different parts of the body work in different ways, but they all repair and reproduce themselves in the same way.

Normally, cells divide in an orderly and controlled manner. However, if for some reason the process gets out of control the cells carry on dividing. There may be too many immature white blood cells in the blood or bone marrow, which may cause a lump or tumour to develop in one or more groups of lymph nodes.

Lymphoma cells generally start to grow in lymph nodes, which are part of the lymphatic system.

Usually non-Hodgkin lymphoma (NHL) affects groups of lymph nodes in one area of the body. However, the lymphoma cells can sometimes spread through the lymphatic system to lymph nodes in other parts of the body. They can also go into the bloodstream, which may carry them to other organs. When the cells reach a new area they may go on dividing and form a new tumour.

There are lymph nodes and lymph vessels throughout the body, so NHL can occur in any part of the body. The most common place is in the lymph nodes in the neck. The next most common places are the lymph glands under the arms (axilla), in the chest, or in the groin or abdomen. It can also occur in organs. In some people NHL can affect the liver, lungs or bone.

Most lymphomas are lymphomas of the B-cells; however there are also some which are lymphomas of the T-cells. These occur more commonly in teenagers and young adults.