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Cancer starts when cells start to grow out of control. Cells in nearly any part of the body can become cancer, and can then spread to other parts of the body. To learn more about how cancers start and spread, see What Is Cancer?

Lymphomas are cancers that start in white blood cells called lymphocytes. There are 2 main types of lymphoma:

• Hodgkin lymphoma (HL)
• Non-Hodgkin lymphoma (NHL)

HL and NHL come from different types of cells. They behave, spread, and respond to treatment differently, so it's important for you to know which one you have.

This information is about Hodgkin lymphoma. To learn about the other type, see Non-Hodgkin Lymphoma.

The lymph system

To understand what Hodgkin lymphoma is, it helps to know 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 control the flow of fluids in the body.

The lymph system is made up mainly of cells called lymphocytes, a type of white blood cell. There are 2 main types of lymphocytes:

• B lymphocytes (B cells): B cells make proteins called antibodies to help protect the body from germs (bacteria and viruses).
• T lymphocytes (T cells): There are many types of T cells. 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.

Hodgkin lymphoma usually starts in B lymphocytes.

Start and spread of Hodgkin lymphoma

Lymph tissue is in many parts of your body, so Hodgkin lymphoma can start almost anywhere. 

The major sites of lymphoid tissue are:

Lymph nodes: Lymph nodes are bean-sized collections of lymphocytes and other immune system cells. They're found throughout the body, including inside the chest, abdomen (belly), and pelvis. They're connected to each other by a system of lymphatic vessels.

Lymph vessels: A network of tiny tubes (a lot like blood vessels) that connect lymph nodes and carry immune cells in a clear fluid called lymph. Lymph is collected from around the body and put into the bloodstream.

Spleen: The spleen is an organ that's under the lower ribs on your left side. The spleen is part of your immune system. It makes lymphocytes and other immune system cells. It also stores healthy blood cells and filters out damaged blood cells, bacteria, and cell waste.

Bone marrow: The bone marrow is the liquid, spongy tissue inside certain bones. New blood cells (including some lymphocytes) are made there.

Thymus: The thymus is a small organ behind the upper part of the breastbone and in front of the heart. It's important for T lymphocyte development.

Adenoids and tonsils: These are collections of lymph tissue in the back of your throat. They help make antibodies against germs that are breathed in or swallowed.

Digestive tract: The stomach, intestines, and many other organs also have lymph tissue.

Although Hodgkin lymphoma can start almost anywhere, most often it starts in lymph nodes in the upper part of the body. The most common sites are in the chest, neck, or under the arms. 

Hodgkin lymphoma most often spreads through the lymph vessels from lymph node to lymph node. Rarely, late in the disease, it can invade the bloodstream and spread to other parts of the body, such as the liver, lungs, and/or bone marrow.

Types of Hodgkin lymphoma

Different types of Hodgkin lymphoma can grow and spread differently and may be treated differently. 

Classic Hodgkin lymphoma

Classic Hodgkin lymphoma (cHL) accounts for more than 9 in 10 cases of Hodgkin lymphoma in developed countries.

The cancer cells in cHL are called Reed-Sternberg cells. These cells are usually an abnormal type of B lymphocyte. Enlarged lymph nodes in people with cHL usually have a small number of Reed-Sternberg cells with a lot of normal immune cells around them. These other immune cells cause most of the swelling in the lymph nodes.

Classic HL has 4 subtypes:

• Nodular sclerosis Hodgkin lymphoma or NSCHL: This is the most common type of Hodgkin disease in developed countries. It accounts for about 7 out of 10 cases. It's most common in teens and young adults, but it can occur in people of any age. It tends to start in lymph nodes in the neck or chest.
• Mixed cellularity Hodgkin lymphoma or MCCHL: This is the second most common type, found in about 4 out 10 cases. It's seen mostly in people with HIV infection. It's also found in children or the elderly . It can start in any lymph node but most often occurs in the upper half of the body.
• Lymphocyte-rich Hodgkin lymphoma: This sub-type isn't common. It usually occurs in the upper half of the body and is rarely found in more than a few lymph nodes.
• Lymphocyte-depleted Hodgkin lymphoma: This is a rare form of Hodgkin disease. It's seen mainly in older people and those with HIV infection. It's more aggressive than other types of HL and likely to be advanced when first found. It's most often in lymph nodes in the abdomen (belly) as well as in the spleen, liver, and bone marrow.

Nodular lymphocyte-predominant Hodgkin lymphoma

Nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL) accounts for about 5% of cases. The cancer cells in NLPHL are large cells called popcorn cells (because they look like popcorn), which are variants of Reed-Sternberg cells. You may also hear these cells called lymphocytic and histiocytic (L&H) cells.

NLPHL usually starts in lymph nodes in the neck and under the arm. It can occur in people of any age, and is more common in men than in women. This type of HL tends to grow more slowly and is treated differently from the classic types.