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Multiple myeloma is one of several diseases that are collectively known as plasma cell dyscrasias. In general, the term myeloma refers to cancer of special types of white blood cells called plasma cells. Plasma cells are important components of the immune system that help the body fight infections caused by microorganisms such as bacteria, viruses, and fungi. Plasma cells are found primarily in the bone marrow and develop from white blood cells called B-lymphocytes. When microorganisms invade the body, B-lymphocytes respond by transforming into plasma cells which, in turn, produce proteins called antibodies that help to destroy the invading microorganisms and, thereby, eradicate the infection. There are five types (classes) of antibodies (immunoglobulins) produced by plasma cells: IgG, IgM, IgA, IgD, and IgE. Each plasma cell produces a specific class of antibodies.

Under normal conditions, the body only produces plasma cells when they are needed to help fight off infections. Once the infection has been eliminated from the body, the old plasma cells die off. Certain genetic mutations can cause plasma cells to become abnormal and continue to divide over and over again and, eventually, form a tumor. These abnormal plasma cells, called myeloma cells are cancer cells that produce a specific type of antibody (monoclonal antibody) called M proteins. The monoclonal antibody that is typically overproduced by the myeloma cells is usually of the IgG or IgA variety. Most commonly, a whole monoclonal antibody is produced, however, in about 20% of cases, only a partial antibody called a light chain is produced by the myeloma cells. Light chains do not remain in the circulation and are found mainly in the urine. The M proteins in patients with multiple myeloma can be detected in the blood and/or urine by specialized techniques known as protein electrophoresis and immunofixation.

Since plasma cells originate from the bone marrow, when plasma cells grow out of control, become abnormal myeloma cells, and produce tumors, the tumors usually develop in the bone marrow. If only a single tumor is present, it is called a solitary plasmacytoma. Typically, however, several tumors can be found throughout the bone marrow and, in these cases, the condition is called multiple myeloma.

In patients with multiple myeloma, the number of myeloma cells in the bone marrow increases signficantly and usually accounts for more than 20% of the total population of cells found in the bone marrow. The abundance of cancerous plasma cells in the bone marrow can lead to complications including:

• Anemia - an abnormally low number of red blood cells in the bloodstream that can cause severe fatigue and weakness.
• Thrombocytopenia - an abnormally low number of platelets in the circulation that can lead to bleeding and/or bruising problems.
• Leukopenia - an abnormally low number of white blood cells in the circulation that increases the risk for developing severe, life-threatening infections.
• Myeloma bone disease - myeloma cells produce a variety of substances (mediators) that stimulate cells called osteoblasts to resorb (dissolve) bone at a much faster rate than cells called osteoblasts can produce new bone. This increased rate of bone resorption in patients with myeloma can cause weak, brittle bones (osteoporosis) and, thereby, increase the risk for developing fractures.