Myeloma is literally an "oma," or tumor, involving the "myelo," or blood-producing cells in the bone marrow. The cells that are affected are plasma cells (a type of white blood cell), which are our antibody- (immunoglobulin-) producing cells. A malignant or cancerous plasma cell is called a myeloma cell. Myeloma is called "multiple" since there are frequently multiple patches or areas in the bones where tumors or lesions have developed. A single lesion is called a solitary plasmacytoma.

Myeloma affects the places where bone marrow is normally active in an adult. This marrow is in the hollow area within the bones of the spine, skull, and pelvic bones, the rib cage, and the areas around the shoulders and hips. The areas usually not affected are the extremities: that is the hands, feet, and lower arm/leg regions. This is very important since the function of these critical areas is usually fully retained.

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