Illustration of Illustration of monoclonal antibodies (blue) being used to treat plasma cell leukaemia (dark purple cells). Plasma cell leukaemia is a rare form of multiple myeloma that affects white blood cells known as plasma cells. The plasma cells become abnormal and cancerous, divide and multiply, and create large quantities of abnormal antibodies. Monoclonal antibodies (mAbs) are proteins secreted by a cloned line of cells which have been engineered to respond to a particular antigen (foreign substance), in this case cancer cells. They can be effective as the body's own immune system will not always recognise cancer cells as foreign.