Lymphoma comprises about 67 subtypes of two related cancers that affect the lymphatic system, Hodgkin lymphoma and non-Hodgkin lymphoma (NHL). There are six types of Hodgkin lymphoma and at least 61 types of non-Hodgkin lymphoma. Hodgkin lymphomas are somewhat different from non-Hodgkin lymphoma in the way they develop, spread and are treated. Lymphoma is the most common blood cancer and the third most common cancer among children.
Lymphoma usually develops when a genetic error, or mutation, occurs within a lymphocyte, causing the abnormal cell to duplicate faster than a normal cell or live longer than normal lymphocyte. Lymphocytes are small white blood cells that play a role in the immune system, which defends the body against foreign invaders, such as bacteria or viruses. Like normal lymphocytes, cancerous lymphocytes can travel in the blood and grow in many parts of the body, including the lymph nodes, spleen, bone marrow, blood or other organs. As a result, while some NHLs are localized in one area of the body, most are present throughout the body by the time of diagnosis.
Although the various types of NHL have some things in common, such as their lymphatic origin, they differ in their appearance under the microscope, their molecular features, their growth patterns, their impact on the body and how they are treated.
Non-Hodgkin lymphoma is divided into two major groups: B-cell lymphomas and T-cell lymphomas. B-cell lymphomas develop from, abnormal B-lymphocytes (“B” because B-lymphocytes come from the bone marrow) and account for 85 percent of all NHLs. T-cell lymphomas develop from abnormal T-lymphocytes (“T” because normally T-lymphocytes spend part of their lifespan in the thymus gland, a small organ in the chest) and account for the remaining 15 percent of NHLs.
Non-Hodgkin lymphoma has grown from being a relatively uncommon disease to being the fifth most common cancer in the United States, nearly doubling in incidence since the early 1970s, and increasing among women since 1991. (Understanding Non-Hodgkin Lymphoma; A Guide for Patients, Survivors and Loved Ones 3rd Edition; Lymphoma Research Foundation)