Multiple myeloma is a rare cancer of the bone marrow. It results from the abnormal growth of plasma cells in the bone marrow. Plasma cells normally produce antibodies. As these abnormal or malignant plasma cells multiply, they produce large quantities of abnormal antibodies. These abnormal antibodies collect in the blood and urine. As the plasma cell tumor grows, it also destroys the bone around it. These events lead to bone pain, kidney damage, and a weak immune system.

Bone Marrow in Adult
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Cancer occurs when cells in the body divide without control or order. Normally, cells divide in a regulated manner. If cells keep dividing uncontrollably when new cells are not needed, a mass of tissue forms, called a growth or tumor. The term cancer refers to malignant growths. These growths can invade nearby tissues. Cancer that has invaded nearby tissues can then spread to other parts of the body.

It is not clear exactly what causes these problems in the cells, but it is probably a combination of genetics and environment.