Signs and symptoms of non-Hodgkin's lymphoma may include:

• Painless, swollen lymph nodes in your neck, armpits or groin

• Abdominal pain or swelling

• Chest pain, coughing or trouble breathing

• Persistent fatigue

• Fever

• Night sweats

• Unexplained weight loss

In most cases, doctors don't know what causes non-Hodgkin's lymphoma. In some cases, it's due to a weakened immune system. But it begins when your body produces too many abnormal lymphocytes — a type of white blood cell.

Normally, lymphocytes go through a predictable life cycle. Old lymphocytes die, and your body creates new ones to replace them. In non-Hodgkin's lymphoma, your lymphocytes don't die, but continue to grow and divide. This oversupply of lymphocytes crowds into your lymph nodes, causing them to swell.

B cells and T cells

Non-Hodgkin's lymphoma can begin in the:

• B cells. B cells fight infection by producing antibodies that neutralize foreign invaders. Most non-Hodgkin's lymphoma arises from B cells. Subtypes of non-Hodgkin's lymphoma that involve B cells include diffuse large B-cell lymphoma, follicular lymphoma, mantle cell lymphoma and Burkitt lymphoma.

• T cells. T cells are involved in killing foreign invaders directly. Non-Hodgkin's lymphoma occurs less often in T cells. Subtypes of non-Hodgkin's lymphoma that involve T cells include peripheral T-cell lymphoma and cutaneous T-cell lymphoma.

Whether your non-Hodgkin's lymphoma arises from your B cells or T cells helps to determine your treatment options.

In most cases, people diagnosed with non-Hodgkin's lymphoma don't have any obvious risk factors. And many people who have risk factors for the disease never develop it.

Some factors that may increase the risk of non-Hodgkin's lymphoma include:

• Medications that suppress your immune system. If you've had an organ transplant, you're more susceptible because immunosuppressive therapy has reduced your body's ability to fight new illnesses.

• Infection with certain viruses and bacteria. Certain viral and bacterial infections appear to increase the risk of non-Hodgkin's lymphoma. Viruses linked to increased non-Hodgkin's lymphoma risk include HIV and Epstein-Barr infection. Bacteria linked to an increased risk of non-Hodgkin's lymphoma include the ulcer-causing Helicobacter pylori.

• Chemicals. Certain chemicals, such as those used to kill insects and weeds, may increase your risk of developing non-Hodgkin's lymphoma. More research is needed to understand the possible link between pesticides and the development of non-Hodgkin's lymphoma.

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