An antigen is anything that can be “seen” by the immune system. There are many different types of antigens. In CAR T-cell therapy, antigens are a substance (or marker) on the outside of a cell that is recognised by the CAR T cell and ‘marks’ the cell for destruction. An example of an antigen (or marker) in CAR T-cell therapy is CD19, found on the outside of both normal and cancerous B cells.