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When a non-Hodgkin lymphoma starts only in the skin (not in other organs or tissues) it is called a skin lymphoma (or cutaneous lymphoma). A lymphoma that starts in lymph nodes or another part of the body and then spreads to the skin is not a skin lymphoma (because it didn’t start there).

What is lymphoma?

Lymphoma is a cancer that starts in white blood cells called lymphocytes, which are part of the immune system. The main types of lymphomas are:

• Hodgkin lymphoma (also known as Hodgkin’s lymphoma, Hodgkin disease, or Hodgkin’s disease)
• Non-Hodgkin lymphoma (also known as non-Hodgkin’s lymphoma, NHL, or sometimes just lymphoma), which includes all skin lymphomas

Lymphocytes are in the lymph nodes (small, bean-sized collections of immune cells throughout the body) and other lymphoid tissues (such as the spleen, bone marrow, and some other organs, including the skin). Lymphomas can start in any of these places.

Hodgkin lymphoma and other types of non-Hodgkin lymphoma are discussed separately.

The lymph system and lymphoid tissue

To understand what lymphoma is, it helps to know something about the lymph system (also known as the lymphatic system). The lymph system is part of the immune system, which helps fight infections and some other diseases. The lymph system also helps fluids move around the body.

Lymphocytes

The lymph system is made up mainly of lymphocytes, a type of white blood cell. The main types of lymphocytes are:

• B lymphocytes (B cells): B cells normally help protect the body against germs (bacteria or viruses) by making proteins called antibodies. The antibodies attach to the germs, marking them for destruction by other parts of the immune system.
• T lymphocytes (T cells): There are several types of T cells, each with a special job. Some T cells destroy germs or abnormal cells in the body. Other T cells help boost or slow the activity of other immune system cells.

Both types of lymphocytes can develop into lymphoma cells. In the skin, T-cell lymphomas are more common than B-cell lymphomas.

Doctors can tell B cells and T cells apart with lab tests that detect certain proteins on their surfaces and certain features of their DNA. These tests also can recognize several stages of B-cell and T-cell development. This can help doctors figure out which type of lymphoma a person has, which can help determine their treatment options.

Lymph tissue

Most lymphocytes are in lymph nodes, which are bean-sized collections of lymphocytes and other immune system cells throughout the body. Lymph nodes are connected to each other by narrow tubes like blood vessels called lymphatics (or lymph vessels), which carry a colorless, watery fluid (lymph) that contains lymphocytes.

Along with the lymph nodes, lymphocytes can be found in the blood and in lymphoid tissues in many other places in the body, including the:

• Spleen
• Bone marrow (the soft, inner parts of certain bones)
• Thymus
• Adenoids and tonsils
• Digestive tract
• Skin
• Other organs

Lymphomas can start in any part of the body that contains lymph tissue.

Some other types of cancer, such as lung cancer or colon cancer, can spread to lymph tissue such as the lymph nodes. But cancers that start in these places and then spread to the lymph tissue are not lymphomas.