What is non-Hodgkin lymphoma?
Non-Hodgkin lymphoma (NHL) is a cancer that starts in lymphocytes. Lymphocytes are cells of the lymphatic system.
The lymphatic system
The lymphatic system works with other parts of your immune system to help your body fight infection and disease. The lymphatic system is made up of a network of lymph vessels, lymph nodes and the lymphatic organs. Lymph vessels carry lymph fluid, which contains lymphocytes and other white blood cells, antibodies and nutrients. Lymph nodes sit along the lymph vessels and filter lymph fluid. The lymphatic organs include the spleen, thymus, adenoids, tonsils and bone marrow.
Lymphocytes develop in the bone marrow from basic cells called stem cells. Stem cells develop into different types of cells that have different jobs. Lymphocytes are types of white blood cells that help fight infection. The main types of lymphocytes are:
- B cells make antibodies to fight bacteria, viruses and other foreign material such as fungi.
- T cells fight infection, destroy abnormal cells and control the immune response.
- Natural killer (NK) cells attack abnormal or foreign cells.
Lymphocytes sometimes change so they no longer grow or behave normally. These abnormal cells can form tumours called lymphomas.
Because lymphocytes are found throughout the lymphatic system, NHL can start almost anywhere in the body. It usually starts in a group of lymph nodes in one part of the body, such as in the neck, above the collarbone, under the arms, in the abdomen or in the groin.
NHL can spread to almost any tissue or organ in the body through the lymphatic system or the bloodstream. Abnormal lymphocytes, or lymphoma cells, may stay in the lymph nodes or form solid tumours in the body. In rare cases, they circulate in the blood.
Types of NHL
There are more than 30 different types of NHL. They are grouped based on the type of lymphocyte they started from. Most types of NHL start in B cells and are called B-cell lymphoma. NHL can also start in T cells, which is called T-cell lymphoma. NHL that starts in NK cells is grouped with T-cell lymphomas.
The different types of NHL look different under a microscope. They also develop and grow differently. The grade of NHL is based on how different, or abnormal, the lymphoma cells look compared to normal lymphocytes. The grade gives doctors an idea of how slowly or quickly the NHL will likely grow and spread. NHL is usually divided into 2 grades:
- Indolent (low-grade) NHL means that the cancer cells are well differentiated. They look and act much like normal cells. These NHLs tend to grow slowly.
- Aggressive (high-grade) NHL means that the cancer cells are poorly differentiated or undifferentiated. They look and act less normal, or more abnormal. These NHLs tend to grow quickly.
World Health Organization (WHO) classification system
Doctors use the WHO classification system to identify the type of NHL. Each type can behave differently and may need different treatments.
The WHO classifies NHL based on the type of lymphocyte (B cell or T cell) that has become cancerous. NHL that starts in natural killer (NK) cells is grouped with T-cell lymphomas.
The WHO divides B-cell and T-cell lymphomas into 2 groups based on the stage of development, or maturation, of the lymphocytes when they became cancerous. Precursor lymphomas develop in immature lymphocytes during the earliest stages of their development. Mature lymphomas (also called peripheral lymphomas) develop in more mature lymphocytes.
The specific type of NHL is based on how the abnormal lymphocytes, or lymphoma cells, look under the microscope, characteristics of chromosomes in the lymphoma cells and if there are certain proteins on the surface of the lymphoma cells.
The WHO includes both lymphomas and lymphocytic leukemias as types of NHL. They both start in lymphocytes, but the lymphoma cells are found in different places in the body. If tumours develop in the lymph nodes or other organs, it is considered a lymphoma. If the lymphoma cells are in the blood or bone marrow and a tumour develops, it is considered a leukemia lymphoma.
Other cancers of the lymphatic system are called Hodgkin lymphoma (HL). The abnormal cells of Hodgkin lymphoma look and behave differently from non-Hodgkin lymphoma cells. Hodgkin lymphomas and non-Hodgkin lymphomas are treated differently.