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

Multiple myeloma is a type of cancer formed by cancerous plasma cells.

Normal plasma cells are an important part of your body’s immune system. Plasma cells are mainly in the bone marrow — the soft, inner part of some bones. These cells make proteins called antibodies that attack and help kill germs.

When plasma cells grow out of control and become cancer cells, they can form a tumor, usually in a bone. If there is only one plasma cell tumor, it is called an isolated (or solitary) plasmacytoma. When there is more than one plasma cell tumor, it is called multiple myeloma.

When the plasma cells grow out of control, they build up in the bone marrow and crowd out the normal cells that make new blood cells. That can lead to low numbers of red blood cells, platelets, and normal white blood cells in the blood. This can cause certain problems, which are discussed in the section “Signs and symptoms of multiple myeloma.”

Another problem is that the antibodies made by the myeloma cells do not help protect your body from infections.

The myeloma cells also signal certain bone cells to dissolve bone. Normally, some cells build up bones and other cells work to dissolve them. But myeloma cells cause too much bone to dissolve. This weakens the bones, and they break easily. When bone dissolves, it releases calcium into the blood. This can lead to high levels of calcium in the blood. This can make a person feel very tired and weak. In extreme cases, it can even cause them to go into a coma.

The myeloma protein can damage the kidneys, leading to problems with kidney function or even kidney failure.

MGUS (monoclonal gammopathy of undetermined significance)

Having many copies of the same antibody is called a monoclonal gammopathy. This is seen in multiple myeloma and some other diseases, but it can also be present and not cause problems. A monoclonal gammopathy that isn’t part of a disease or causing a problem is called monoclonal gammopathy of undetermined significance or MGUS. There may be some extra plasma cells in the bone marrow, but they do not form a tumor or cause any problems. They do not weaken the bones. But with time, MGUS can turn into multiple myeloma or another disease. People with MGUS don’t need treatment, but they are watched closely to see if they get a disease that does need to be treated (like multiple myeloma).

Last Medical Review: 05/22/2014
Last Revised: 01/19/2016