The CD3 protein is a critical signaling component of the T-cell receptor complex. T-cell engager (TCE) drugs, such as bispecific antibodies, are designed with one end binding to a tumor-associated antigen on cancer cells and the other end precisely engaging the CD3 protein. This binding forcibly recruits and anchors T cells in close proximity to cancer cells, bypassing normal antigen presentation and directly activating T cells via CD3, thereby efficiently killing tumors. CD3 serves as the core switch in this class of drugs for activating T-cell immune attacks.