Plasma cells and other white blood cells are part of the immune system, which helps the body protect itself from infection and disease. All white blood cells develop in the bone marrow, the spongy tissue that fills the centers of most bones. Some of these white blood cells remain in the bone marrow, and some migrate to other parts of the body. Some of the white blood cells, whether in the marrow or other parts of the body, develop into plasma cells when the immune system needs them to fight infection and disease. Plasma cells are important to the immune system because they produce antibodies.
Antibodies are proteins that move throughout the blood stream to fight foreign, harmful substances.
Each type of plasma cell produces a specific antibody to fight a specific foreign substance. Since the body has many different types of plasma cells, it can respond to, or fight against, many different foreign substances. When plasma cells become cancerous, they reproduce uncontrollably. The tremendously increased number of plasma cells in multiple myleoma crowd out healthy red and white blood cells, preventing them from functioning as effectively as they should. The malignant plasma cells produce osteoclast activating factors which cause calcium to be leached out of the bones.