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

The immune system is composed of several types of cells that work together to fight infections and other diseases. Lymphocytes (lymph cells) are the main cell type of the immune system. There are 2 types of lymphocytes: T cells and B cells.

When B cells respond to an infection, they mature and change into plasma cells. Plasma cells make the antibodies (also called immunoglobulins) that help the body attack and kill germs. Lymphocytes can be found in many areas, such as lymph nodes, the bone marrow, and the bloodstream. Plasma cells, however, are mainly found in the bone marrow. Bone marrow is the soft tissue found inside some hollow bones.

When plasma cells become cancerous and grow out of control, they can produce a tumor called a plasmacytoma. These tumors generally develop in a bone, but they are also rarely found in other tissues. If there is only a single plasma cell tumor, it is called an isolated (or solitary) plasmacytoma. When many plasma cell tumors are found throughout the bones, it is called multiple myeloma.

In multiple myeloma, the overgrowth of plasma cells in the bone marrow can crowd out the normal blood-forming cells. This can cause anemia -- a shortage of red blood cells. Anemia causes people to become pale, weak, and fatigued. Multiple myeloma can also cause the level of platelets in the blood to become low (called thrombocytopenia). This can lead to increased bleeding and bruising. Another condition that can be seen is leukopenia -- a shortage of normal white blood cells. This can lead to difficulty fighting infections.

The myeloma cells also react with the cells that help keep the bones strong. There are 2 major kinds of bone cells that normally work together to keep bones healthy and strong. The cells that lay down new bone are called osteoblasts. The cells that break down old bone are called osteoclasts. Myeloma cells make a substance that tells the osteoclasts to speed up the dissolving of bone. Since the osteoblasts do not get a signal to put down new bone, old bone is broken down without new bone to replace it. This makes the bones weak and they break easily. Fractured bones are a major problem in people with myeloma.

The abnormal plasma cells do not protect the body from infections. As mentioned before, normal plasma cells produce antibodies that attack germs. For example, if you developed pneumonia, normal plasma cells would produce antibodies that specifically attack and kill the bacteria that were causing the illness. In multiple myeloma, the myeloma cells crowd out the normal plasma cells, so that the antibodies to fight the infection aren't made. The antibody made by the myeloma cells does not help fight infections. That is because the myeloma cells are just many copies of the same plasma cell -- all making copies of the same exact antibody. Having many copies of the same antibody is known as a monoclonal gammopathy. This condition can be found with a blood test.

Sometimes, someone has a monoclonal gammopathy but does not have multiple myeloma. This condition is called monoclonal gammopathy of undetermined significance (MGUS). In some cases, people with MGUS go on to develop multiple myeloma.

Monoclonal gammopathy of undetermined significance

In monoclonal gammopathy of undetermined significance (MGUS), abnormal plasma cells produce excess amounts of antibody protein. However, these plasma cells do not form an actual tumor or mass and do not cause any of the other problems seen in multiple myeloma. MGUS usually does not affect a person's health. In particular, it doesn’t cause weak bones or anemia. It is found because a routine blood test finds a high level of protein in the blood and further testing shows the protein is immunoglobulin. In MGUS, less than 10% of the cells in the bone marrow are plasma cells.

Over time, many people with MGUS eventually develop multiple myeloma, lymphoma, or a disease called amyloidosis. The rate of this happening is about 1% per year. The risk of this happening is higher in people whose protein levels are particularly high. Patients 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).

Recently, scientists have studied the genes of the plasma cells in patients with MGUS. They found that the genetic make-up of these plasma cells resembles myeloma plasma cells more than it resembles normal plasma cells. This suggests that these cells are truly malignant, not just slow growing. Because, in general, people with MGUS are elderly, they may not live long enough for it to transform into myeloma.

Solitary plasmacytomas

This is another type of abnormal plasma cell growth. Rather than multiple tumors in different locations as in multiple myeloma, there is only 1 tumor, hence the name solitary plasmacytomas.

Most often, a solitary plasmacytoma develops in a bone, where it may be called an isolated plasmacytoma of bone. When a plasmacytoma starts in other tissues (such as the lungs or the lining of the sinuses, throat, or other organs), it is called an extramedullary plasmacytoma. Solitary plasmacytomas are most often treated with radiation therapy. Sometimes surgery may be used for a single extramedullary plasmacytoma. As long as no other plasmacytomas are found later on, the patient's outlook is usually excellent. However, since many people with a solitary plasmacytoma will develop multiple myeloma, these people are watched closely for signs of this disease.

Last Medical Review: 02/13/2009
Last Revised: 05/12/2009