Multiple myeloma (MM) is technically a malignant proliferation of plasma cells, which are a type of antibody-producing white blood cells. The name "multiple myeloma" was coined in 1873 because the disease was thought to always involve a multiplicity of distinct sites within the bone marrow. We now know this is not always the case. But multiple myeloma most typically (in 90 percent of cases) does involve the bone marrow, where immune cells are produced. The National Cancer Institute has promoted the use of the term "plasma cell neoplasm," because of the inherent inaccuracy in the name "multiple myeloma." However, the name multiple myeloma is time-tested and very persistent, and that is how we shall continue to refer to it. (You may also see it referred to as plasmacytoma, myelomatosis or Kahler's disease.)

The immune system is essentially made up of two types of cells, B cells and T cells.   Both T and B cells are members of a class of cells called lymphocytes. Plasma cells are B cells that have responded to an infection and have become immunologically mature and capable of producing antibodies. In multiple myeloma, malignant plasma cells accumulate in several areas, but particularly within the bone marrow, where they eventually crowd out and interfere with the blood-cell producing function of the bone marrow, causing anemia. Often the accumulation of malignant plasma cells within the bone marrow also leads to a shortage of platelets (the cells that are responsible for blood clotting) and to a disruption in the normal functioning of the immune system. Patients with multiple myeloma may therefore exhibit not only anemia but also bruising, abnormal bleeding, and susceptibility to infection. The first symptom is usually bone pain, which can be associated with compression fractures of the spine. There can also be pathologic fractures of the long bones, caused by the disease having weakened the bones.

MM is considered highly treatable but rarely curable by conventional medicine. Like several other kinds of hematological cancer (for example, chronic lymphocytic leukemia) it is very often characterized by the presence in the urine or the blood of a particular kind of monoclonal immunoglobulin (Ig) known as M protein, or M-component, which gives rise to the characteristic Bence-Jones protein in the urine. MM is one of a related group of diseases that also includes such entities as "solitary" myelomas of the bone and Waldenström's macroglobulinema. MM is the most common of these disorders.

There are several different types of MM. At the least aggressive end of the scale there is smoldering multiple myeloma (SMM), in which patients have myeloma cells in the bone marrow and a large amount of Bence-Jones protein, but do not have the skeletal lesions, anemia, or kidney failure that otherwise characterize the disease. These patients generally have no symptoms. Their disease may be discovered in the course of other medical procedures. Similarly, those with slight abnormalities in the blood picture may be designated as having monoclonal gammopathy of unknown significance (MGUS). This, too, is a very early and indolent variant, but a significant proportion of patients with MGUS progress to full-blown myeloma at a later date, and therefore must be carefully watched.

Other manifestations of MM include plasma cell leukemia, osteosclerotic myeloma, solitary plasmacytosis and extramedullary plasmacytoma.