Illustration of monoclonal antibodies (blue) attacking cancerous white blood cell precursors (dark pink) in a case of chronic lymphoblastic leukaemia (CLL). Surrounding cells include healthy red blood cells (red discs) and white blood cells (light pink). CLL is a cancer in which excess white blood cell precursors called lymphoblasts are produced in the bone marrow. The chronic form develops very slowly, and symptoms may not appear for years. Treatment can involve immunotherapy, chemotherapy and radiotherapy. 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 bodys own immune system will not always recognise cancer cells as foreign.