Illustration of chimeric antigen receptor (CAR) T cells (blue) attacking cancerous white blood cell precursors (purple) in a case of acute lymphoblastic leukaemia (ALL) in a blood vessel. Healthy red blood cells (discs) can be seen in the background. ALL is a rare cancer in which excess white blood cell precursors called lymphoblasts are produced in the bone marrow. It is the most common form of leukaemia in children, but affects adults as well. CAR-T therapy is a type of immunotherapy that genetically modifies a patients own T cells, a form of white blood cell, to recognise and destroy cancer cells.