The immune system is composed of several types of cells that work together to fight off infections and other diseases. When B-cells respond to an infection, they mature and change into plasma cells. When plasma cells grow excessively they can produce a tumor. These tumors can grow in several sites, particularly in the soft middle parts of bone called the bone marrow. When these tumors grow in multiple sites they are referred to as multiple myeloma.
Plasma cells are the cells responsible for forming antibodies against bacteria and foreign proteins.
For reasons that are unclear, these cells lose their ability to respond to controlling signals from a hierarchy of immune cells. Plasma cells are made by white blood cells called lymphocytes. The plasma cells make antibodies, which fight infection and other harmful things in the body. When these cells become cancer, they may make too many antibodies and a substance called M-protein is found in the blood and urine. There are several types of plasma cell neoplasms.