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Non-Hodgkin lymphoma is caused by a change (mutation) in the DNA of a type of white blood cell called lymphocytes, although the exact reason why this happens isn't known.

DNA gives cells a basic set of instructions, such as when to grow and reproduce.

The mutation in the DNA changes these instructions, so the cells keep growing. This causes them to multiply uncontrollably.

The abnormal lymphocytes usually begin to multiply in one or more lymph nodes in a particular area of the body, such as your neck, armpit or groin.

Over time, it's possible for the abnormal lymphocytes to spread into other parts of your body, such as your:

• bone marrow
• spleen
• liver
• skin
• lungs
• stomach
• brain

But in some cases non-Hodgkin lymphoma starts growing in an organ, rather than starting in a lymph node and spreading to an organ.

While the cause of the initial mutation that triggers non-Hodgkin lymphoma is unknown, a number of factors can increase your risk of developing the condition.

These include having:

• a medical condition that weakens your immune system, such as HIV
• medical treatment that weakens your immune system – for example, taking medicine to suppress your immune system after an organ transplant
• an autoimmune condition (a condition caused by problems with the immune system), such as rheumatoid arthritis, lupus or Sjögren's syndrome
• the Epstein-Barr virus – a common virus that causes glandular fever
• the human T-cell lymphotropic virus (HTLV)
• a Helicobacter pylori infection – a common bacterial infection that usually infects the lining of the stomach and small intestine
• received chemotherapy or radiotherapy for an earlier cancer
• coeliac disease – an adverse reaction to gluten that causes inflammation of the small bowel

Non-Hodgkin lymphoma isn't infectious and isn't thought to run in families, although your risk may be slightly increased if a close relative (such as a parent or sibling) has had lymphoma.

Non-Hodgkin lymphoma can occur at any age, but a third of cases are diagnosed in people over 75. It's slightly more common in men than women.