Hodgkin lymphoma is a cancer that begins in cells of the immune system. The immune system fights infections and other diseases.
Hodgkin lymphoma can start almost anywhere. Usually, it's first found in a lymph node above the diaphragm, the thin muscle that separates the chest from the abdomen, but Hodgkin lymphoma also may be found in a group of lymph nodes. Sometimes it starts in other parts of the lymphatic system.
Hodgkin lymphoma begins when a lymphocyte (usually a B cell) becomes abnormal. The abnormal cell is called a Reed-Sternberg cell.
The Reed-Sternberg 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.