Multiple myeloma is a type of cancer formed by malignant plasma cells. Normal plasma cells 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 two types of lymphocytes: T cells and B cells.

When B cells respond to an infection, they mature and change into plasma cells. Plasma cells produce and release proteins called antibodies (or immunoglobulins) to attack and help kill disease-causing germs such as bacteria.

When plasma cells grow out of control, they can produce a tumor. These tumors can grow in several sites, particularly in the soft middle parts of bone marrow. When these tumors grow in multiple sites, they are referred to as multiple myeloma. The overgrowth of plasma cells can interfere with the normal blood-forming functions of the bone marrow. This can result in a shortage of red blood cells and a condition called anemia. Anemia will cause fatigue. A shortage of blood platelets (cells that seal damaged blood vessels) can also occur. This can lead to excessive bleeding after cuts or scrapes. Another problem caused by an excess of plasma cells is leukopenia, a condition in which there is a shortage of normal infection-fighting white blood cells. A shortage of these cells causes decreased resistance to infections.

The abnormal plasma cells do not protect the body from infections. As mentioned before, normal plasma cells produce antibodies that attack specific infectious agents. For example, if you developed pneumonia normal plasma cells would produce antibodies that specifically attack and kill this type of bacteria. However, the antibodies produced by myeloma cells are not helpful in fighting infections.

Multiple myeloma is not the only disease involving excessive plasma cell growth. Two other forms of plasma cell disease are monoclonal gammopathy of undetermined significance (MGUS) and solitary plasmacytoma.

Solitary plasmacytoma is another category of plasma cell growth. Rather than multiple tumors in different locations as in multiple myeloma, there is only 1 tumor, hence the name "solitary" plasmacytomas.

Solitary plasmacytomas develop in bone marrow, or they may start in tissues other than bone marrow (such as the lungs or the lining of the sinuses, throat, or other organs). This is called extramedullary disease meaning outside the bone marrow. These tumors are treated by radiation therapy and/or sometimes with surgery. Their prognosis (outlook for recovery or survival) is usually excellent if no other plasmacytomas are found later on. However, most people with solitary plasmacytoma eventually develop multiple myeloma, especially if the plasmacytoma was in bone, and therefore need frequent examinations and tests to detect this progression as early as possible. Early treatment may lead to an improved outcome.