Hodgkin lymphoma is a cancer that starts in a white blood cell called a B-lymphocyte. Healthy B-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 B-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.

However, in Hodgkin lymphoma, some of the B-lymphocytes are no longer healthy and do not fight infection. Instead, the abnormal (cancerous) lymphocytes begin to grow out of control, causing the lymph nodes to get bigger. These cancerous lymphocytes can also spread outside the lymph nodes. The cancerous B-lymphocyte of Hodgkin lymphoma is often surrounded by a large number of normal immune cells. When a lymph node is affected by Hodgkin lymphoma, it typically becomes much larger than it would with an infection. As Hodgkin lymphoma advances, it may spread beyond the lymph node to the bone marrow, liver, or lungs.

Children and adults can get Hodgkin lymphoma. In the United States, Hodgkin lymphoma is most frequently diagnosed in young adults between 20 and 34 years old. About 10 percent to 15 percent of cases occur in children younger than 20 years old.