Normally, your white blood cells (lymphocytes) go through a predictable life cycle. Old lymphocytes die, and your body creates new ones to replace them. But in non-Hodgkin's lymphoma, your body produces abnormal lymphocytes that continue to divide and grow uncontrollably. This oversupply of lymphocytes crowds into your lymph nodes, causing them to swell.

Doctors don't know what exactly causes non-Hodgkin's lymphoma. But researchers believe that activation of certain abnormal genes may be involved in the development of all cancers, including lymphomas.

Lymphocytes exist either as B cells, those that originate in the bone marrow, or T cells, which derive from the thymus gland. B cells fight infection by producing plasma cells, which in turn produce antibodies that neutralize foreign invaders. T cells are involved in killing foreign invaders directly. About 85 percent of non-Hodgkin's lymphomas occur in B cells. The rest arise in T cells.

Non-Hodgkin's lymphoma generally involves the presence of cancerous cells in your lymph nodes, but the disease can also spread to other parts of your lymphatic system. These include the lymphatic vessels, tonsils, adenoids, spleen, thymus and bone marrow. Occasionally, non-Hodgkin's lymphoma involves organs outside of the lymphatic system.

In most cases, people diagnosed with non-Hodgkin's lymphoma don't have any obvious risk factors. Some factors that may contribute to your risk of non-Hodgkin's lymphoma include:

• Age. Non-Hodgkin's lymphoma can occur at any age, but the risk increases with age. It's most common in people in their 60s.

• Immunosuppression. If you've had an organ transplant, you're more susceptible because immunosuppressive therapy has impaired your immune mechanisms.

• Infection. Having acquired immunodeficiency syndrome (AIDS), in which your immune system is progressively weakened, also places you at higher risk. In Africa, infection with the parasite that causes malaria or the Epstein-Barr virus appears to raise the risk of a particular type of non-Hodgkin's lymphoma, called Burkitt's lymphoma. An infection with the bacterium Helicobacter pylori, which is known to cause ulcers, can cause an immune system response that raises your risk of non-Hodgkin's lymphoma, particularly in the stomach.

• Chemicals. Certain chemicals, such as those used to kill insects and weeds, may increase your risk of developing non-Hodgkin's lymphoma. Long-term use of dark colored hair dyes has been linked to an increased risk of the disease.