Non-Hodgkin lymphoma is cancer that begins in cells of the immune system. The immune system fights infections and other diseases.
The lymphatic system is part of the immune system. The lymphatic system includes the following:
Because lymphatic tissue is in many parts of the body, lymphoma can start almost anywhere. Usually, it's first found in a lymph node.
Non-Hodgkin lymphoma begins when a lymphocyte (usually a B cell) becomes abnormal. The abnormal cell divides to make copies of itself. The new cells divide again and again, making more and more abnormal cells. The abnormal cells don't die when they should. They don't protect the body from infections or other diseases. The buildup of extra cells often forms a mass of tissue called a growth or tumor.
See the Staging section for information about lymphoma that has spread.
From What You Need To Know About Non-Hodgkin Lymphoma, National Cancer Institute