Structure and mechanism of CAR-modified T cells. T cells are engineered to express a CAR, which links an extracellular antibody domain (scFv) to intracellular T-cell signaling domains, the CD3 zeta cytoplasmic domain and a costimulatory domain (CD28 or 4-1BB). Once engaged by their target, CARs activate a cytotoxic T-cell response that kills the bound antigen-expressing cell.