Multiple myeloma is a type of cancer characterized by malignant plasma cells that reproduce uncontrollably. A plasma cell is a type of white blood cell that is part of the immune system. When the immune system needs to fight infection and disease, some of the white blood cells develop into plasma cells. All white blood cells develop in the bone marrow, the tissue that fills the centers of most bones; some of these white blood cells remain in the bone marrow, and others migrate to other parts of the body.
Plasma cells are important to the immune system because they produce antibodies. Antibodies are proteins that move throughout the bloodstream to fight foreign, harmful substances. Each type of plasma cell produces a specific antibody to fight a specific foreign substance. The body has many different types of plasma cells, and this allows it to respond to many different foreign substances.
The malignant myeloma plasma cells produce excessive amounts of abnormal antibodies called monoclonal immunoglobulin, also known as M protein. Because myeloma cells produce large amounts of M protein, analysis of blood or urine samples from patients with multiple myeloma usually reveals an “M spike,” which is monitored as an indicator of disease activity and progression.
In multiple myeloma, the tremendously high number of plasma cells crowd out healthy red and white blood cells, preventing them from functioning as effectively as they should. This can lead to anemia and problems with infection, bruising, and bleeding. The malignant plasma cells produce proteins that cause calcium to be leached out of the bones, which can result in dangerously high levels of calcium in the bloodstream (hypercalcemia) and holes in bones (osteolytic lesions). The osteolytic lesions weaken bones and can lead to vertebral collapse or spontaneous fractures. Excessive amounts of M protein produced by the malignant plasma cells can overload the kidneys, leading to abnormal kidney function and even renal failure. Sometimes malignant plasma cells congregate in clumps, causing isolated tumors, or plasmacytomas.