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Non-Hodgkin lymphoma (which is also known as B-cell and T-cell lymphomas) is a cancer of the lymphatic system. The lymphatic system is a network of vessels and glands spread throughout the body. It is also part of the immune system. Clear fluid called lymph flows through the lymphatic vessels and contains infection-fighting white blood cells known as lymphocytes.
In lymphoma, these lymphocytes start to multiply in an abnormal way. A lot of them appear in certain parts of the lymphatic system, such as the lymph nodes (glands). The affected lymphocytes lose their infection-fighting properties making you more vulnerable to infection.
The usual way to know that a person has non-Hodgkin lymphoma is to do a biopsy (testing a sample of affected lymph node tissue).