Non-Hodgkin lymphoma (NHL) is a form of cancer that starts in a white cell called a lymphocyte. Healthy lymphocytes are part of the immune system. They fight infections and illnesses. They are often found in bean-sized lymph nodes or in other lymphoid tissues such as the spleen. Because lymphocytes and other immune cells are present in lymph nodes, it is normal for the lymph nodes to get bigger when the body is fighting an infection.

In NHL, some of the lymphocytes are no longer healthy and do not fight infection. Instead, the abnormal (cancerous) lymphocytes divide rapidly, crowding out the normal cells and causing the lymph nodes to get bigger. As the disease advances, it may spread to the bone marrow, central nervous system, liver, spleen and reproductive organs.