Multiple myeloma is a type of cancer formed by cancerous plasma cells. Normal plasma cells are found in the bone marrow and are an important part of the body's immune system.

The immune system is made up of several types of cells that work together to fight infections. Lymph cells (called lymphocytes) are the main type of cell in the immune system. There are 2 types of lymph cells: T cells and B cells.

When B cells respond to an infection, they change into plasma cells. The plasma cells are mainly in the bone marrow—the soft, inner part of some bones. The plasma cells make proteins called antibodies that attack and help kill germs.

When plasma cells grow out of control, they can form a tumor, usually in the bone marrow. This type of tumor is called a myeloma, and if there are many tumors they are called multiple myeloma. If there is only one tumor, it is called solitary plasmacytoma. In many cases, this single tumor will go on to become multiple myeloma.

Having too many plasma cells can cause problems in the bone marrow, where all blood cells are made. The bone marrow may not be able to make enough red blood cells, platelets, or normal white blood cells. This can cause problems such as the following:

Anemia: A shortage of red blood cells, which can cause you to be pale, weak, and tired.

Bruising or bleeding: Cells called platelets help stop bleeding. Without enough platelets, there can be a lot of bruising or bleeding.

Leukopenia: A shortage of white blood cells, which can make it very hard for the body to fight infections.

Another problem is that the myeloma cells do not help protect the body from infections. They make antibodies, like normal plasma cells do, but these antibodies do not work to kill germs.

The myeloma also reacts with cells that work to keep the bones strong. Normally, some cells build up bones and other cells work to dissolve them. Together, they give bone its proper shape. But myeloma cells cause too much bone to dissolve. Then the body does not get a signal to make new bone. Old bone is broken down without new bone to replace it. This makes the bones weak and easily broken.

MGUS

Sometimes, before multiple myeloma develops, there will be an overgrowth of plasma cells. This is called MGUS (monoclonal gammopathy of undetermined significance). The plasma cells do not form a tumor or cause symptoms. They do not cause the bones to weaken. But with time, MGUS will often turn into cancer 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: 03/13/2009
Last Revised: 05/06/2009