Myeloma occurs when abnormal plasma cells of the bone marrow grow in an uncontrolled way.
Plasma cells develop in the bone marrow from B lymphocytes, a type of white blood cell. Plasma cells make antibodies that help the body fight infection.

Myeloma occurs when plasma cells grow abnormally and become myeloma cells. Myeloma cells build up in the bone marrow and form tumours in one or many bones of the body. Myeloma cells also crowd the bone marrow, preventing it from making healthy blood cells.

When myeloma cells are found in many bones of the body, this is called multiple myeloma. Multiple myeloma is the most common form of myeloma.

When myeloma cells are only found in only one part of the body, this is called a plasmacytoma. Plasmacytomas may form in bone or in soft tissues such as the throat or tonsils.