Cancer starts when cells in the body change (mutate) and grow out of control. To help you understand what happens when you have cancer, let's look at how your body works normally. Your body is made up of tiny building blocks called cells. Normal cells grow when your body needs them. They die when your body doesn't need them any longer.

Cancer is made up of abnormal cells that grow even though your body doesn't need them. In most cancers, the abnormal cells grow to form a lump or mass called a tumor. If cancer cells are in your body long enough, they can grow into, or invade, nearby areas. They can even spread to other parts of your body. This is called metastasis.

Non-Hodgkin lymphoma is cancer that starts in the lymphatic (lymph) system. Certain cells in the lymph system, called lymphocytes, grow out of control. The lymphoma cells, or changed (mutated) lymphocytes, collect in the lymph nodes and form a tumor. Sometimes tumors form in the spleen or in other lymph organs, like the thymus or tonsils. It can also sometimes affect the skin.

Non-Hodgkin lymphoma is not the same as Hodgkin lymphoma. With Hodgkin lymphoma, cancer cells make up only a small part of the cells in a cancerous lymph node. The rest of the cells are normal immune cells. Hodgkin lymphoma cells are also usually special cells called Reed-Sternberg cells. In non-Hodgkin lymphoma, cancer cells make up almost all of a lymph node, and there are no Reed-Sternberg cells in it.

Hodgkin and non-Hodgkin lymphoma also differ in the way they spread and in how they are treated.

The lymphatic system is part of the immune system. It's spread all through the body and helps fight infection. It also helps maintain fluid balance in different parts of the body. The lymphatic system includes:

• Lymphocytes. These are a type of white blood cell. They fight infection and disease.
• Lymph. This is a clear fluid that contains lymphocytes.
• Lymph vessels. These are tiny tubes. The network of lymph vessels carries lymph fluid all over the body and back to the bloodstream.
• Lymph nodes. These are small organs about the size of a pea. They're found in clusters in the underarms, groin, neck, chest, belly (abdomen), and other parts of the body. They filter the lymph fluid as it moves through the body.
• Other organs and body tissues. The lymphatic system includes the bone marrow where blood is made. It also includes the spleen, thymus, tonsils, adenoids, and the digestive tract.

There are 2 main kinds of lymphocytes:

• B lymphocytes (B cells). B cells fight germs like bacteria by making antibodies. These antibodies attach to the bacteria and attract other cells that fight the bacteria. The antibodies also attract proteins from the blood to help kill bacteria.
• T lymphocytes (T cells). T cells protect the body from fungi, viruses, and some bacteria. T cells are able to find viral proteins in a virus-infected cell and then destroy the infected cell. T cells also send out proteins called cytokines. These proteins signal other types of white blood cells to help fight the infection. T cells can kill some cancer cells, too.

Cancer that starts in B cells, or B-cell lymphoma, is much more common than T-cell lymphoma. About 85% to 90% of people with non-Hodgkin lymphoma in the U.S. have a B-cell lymphoma.

Lymphoma can start in any part of your lymphatic system. It can then spread to other parts of your body. Lymphoma can also spread to the bone marrow and to other organs. It can spread in different ways. It depends on the type of lymphoma and where it first started.

Lymphoma that starts in an organ that's not a lymph node, such as the stomach or spleen, is called extranodal lymphoma.

Lymphomas can be grouped in different ways. They can be grouped by the type of lymphocytes they start in: B cells or T cells. Another way to group them is based on how fast they grow, such as:

• Indolent (low-grade) lymphoma. These types grow and spread slowly. They tend to cause few symptoms, at least at first. These lymphomas may not need to be treated right away. And they often respond well to treatment, so people with this type can often live for a long time. Still, indolent lymphomas tend to be much harder to cure than faster-growing (aggressive) lymphomas. Follicular lymphoma is the most common type of indolent lymphoma in the United States.
• Aggressive (high-grade) lymphoma. These types tend to grow and spread quickly. They usually need to be treated right away. Even though they grow quickly, these lymphomas often respond well to treatment. Some of them can be cured.

There are many different types of non-Hodgkin lymphoma. They grow at different rates and are treated in different ways. Finding out which type of non-Hodgkin lymphoma you have is very important in choosing the best treatment.

Common types of B-cell lymphoma include:

• Diffuse large B-cell lymphoma (DLBCL). DLBCL is the most common type of non-Hodgkin lymphoma in the U.S. It makes up almost 1/3 of all lymphomas. It's a fast-growing or aggressive type of lymphoma. About 3 in 4 people have no signs of lymphoma after treatment, and many people with DLBCL can be cured. The outcome or prognosis is best for people with lymphoma in only 1 part of the body.
• Follicular lymphoma. This type makes up about 1 in 5 lymphomas. It's an indolent or slow-growing type of lymphoma. Most of the time, it's found in many lymph nodes throughout the body and in the bone marrow. This lymphoma tends to grow slowly and often responds well to treatment. But it's hard to cure. This type of lymphoma may not need to be treated at first. Treatment may be delayed until the lymphoma causes problems.
• Chronic lymphocytic leukemia (CLL) and small lymphocytic lymphoma (SLL). These related types are named based on where the lymphoma cells are found. CLL is mostly in the blood and bone marrow. SLL is mainly in the lymph nodes. Both are slow-growing diseases. These lymphomas are often not curable with standard types of treatment. But most people can live with these types of lymphoma for a long time.
• Extranodal marginal zone B-cell lymphomas is also known as mucosa-associated lymphoid tissue (MALT) lymphomas. This is the most common type of marginal zone lymphomas. They start outside of the lymph nodes. Most start in the stomach and are linked to an infection by Helicobacter pylori bacteria. MALT lymphomas can also start in the skin, lungs, thyroid, salivary glands, spleen, and the tissues around the eye. When it's in the stomach, this lymphoma can often be cured by giving antibiotics to kill the H. pylori bacteria that cause it.
• Mantle cell lymphoma. This type accounts for about 1 in 20 lymphomas. It's much more common in men than women. In most cases, it's widespread when diagnosed in lymph nodes, bone marrow, the gastrointestinal system, and often the spleen. This lymphoma does not grow very fast. But it can be hard to treat.
• Primary mediastinal B-cell lymphoma. This is a subtype of DLBCL. It accounts for about 1 in 50 lymphomas. It's mostly found in young women. This lymphoma tends to form a large mass in the area behind the chest bone, called the mediastinum. It's a fast-growing lymphoma, but it's treatable.
• Burkitt lymphoma. This type is rare in adults. It's similar to Burkitt-like lymphoma. Most people with it are male. This lymphoma grows very quickly. But it can be treated and more than half of people with Burkitt lymphoma can be cured with chemotherapy.
• Lymphoplasmacytic lymphoma or Waldenstrom macroglobulinemia. This is a rare, slow-growing type of lymphoma. It's found mainly in the bone marrow, lymph nodes, and spleen. This type of lymphoma can't be cured. But people can live with it for many years.
• Hairy cell leukemia. Despite its name, this type is often considered to be a lymphoma. Only about 700 people in the U.S. are diagnosed with this type every year. It's named for the hairy look of its cells. This lymphoma is typically found in the bone marrow, spleen, and blood. It occurs more often in men. It tends to be slow-growing and is often curable.
• Primary central nervous system lymphoma. This type usually grows in the brain. It may also be found in the spinal cord and the tissues around it, as well as in the eye (called primary intraocular lymphoma). Over time, it becomes widespread in the central nervous system. This type of lymphoma is more common in people with immune system problems, like AIDS.

Common types of T-cell lymphoma include:

• T-lymphoblastic lymphoma/leukemia. This is a rare type of lymphoma. It is most common in teens or young adults. Males are most affected. It is fast-growing, with a good chance of cure if it didn't already spread to the bone marrow when first diagnosed.
• Angioimmunoblastic T-cell lymphoma. This fast-growing type tends to occur in the lymph nodes, bone marrow, spleen, and liver. It causes infections, fever, weight loss, and skin rashes. It's not clear if this lymphoma is curable. It tends to come back after treatment.
• Extranodal natural killer/T-cell lymphoma, nasal type. This aggressive lymphoma often grows in the upper airway passages, such as the nose and upper throat. It can also grow into the skin and digestive tract. It's very rare in the U.S.
• Enteropathy-associated intestinal T-cell lymphoma. This fast-growing type occurs in people with sensitivity to gluten, the main protein in wheat flour. Celiac disease (or gluten-sensitive enteropathy) that started in adulthood is linked with this type of lymphoma. It often grows into the lining of the intestines, and it can be hard to treat.
• Anaplastic large cell lymphoma (ALCL). The primary cutaneous type is rare and slow-growing. It starts in the lymph nodes and spreads to the skin. Systemic ALCL is more aggressive and can affect other organs. It's rare and more common in children than adults. Treatment often works well for both types of ALCL. A third type is called breast implant-associated ALCL. It's also rare. Treatment involves removing the implant.
• T-cell lymphoma/leukemia. This very rare disease can be called lymphoma or leukemia. It depends on how much it's growing in the bone marrow. Leukemia involves more bone marrow than lymphoma. This type linked to infection with the human T-cell lymphotropic virus 1. It tends to be fast-growing, though some subtypes can grow slowly.
• Cutaneous T-cell lymphoma (mycosis fungoides, Sezary syndrome). This starts in the skin. It accounts for about 1 in 20 lymphomas. It often starts as a patchy, red rash. It can grow into solid, raised tumors that can grow into lymph nodes and organs, such as the liver and spleen. With Sezary syndrome, the lymphoma cells are found in the blood. Mycosis fungoides involves the skin.
• Peripheral T-cell lymphoma, not otherwise specified. This term is used to describe aggressive T-cell lymphoma that doesn't fit in any of the other groups. Most people with this type are in their 60s. It tends to grow quickly and be widespread.

If you have questions about non-Hodgkin lymphoma, talk with your provider. Your provider can help you understand more about this cancer.