CD28 and neoplasm: When a CAR is transduced into human T cells, the antibody fragment is expressed on the surface of the engineered T cells to recognize a tumor antigen expressed on tumor cells, while the CD28-CD3ζ or 4-1BB-CD3ζ domain delivers a stimulatory signal once the antibody binds to a tumor antigen, activating CAR-T cells to attack the tumor.