Back on the scene: Advances and challenges in CD3-related drugs in tumor therapy

•CD3 regulates a variety of T cell functions and is essential for immune activation.•CD3 monoclonal antibodies are widely used in the treatment of autoimmune diseases.•T cell redirection by bispecific antibody provides new hope for tumor therapy.•Only three CD3-related small molecules have been reported so far. CD3 molecules are mainly distributed on the membrane of mature T cells. They are involved in T cell antigen recognition, signal transduction, and regulation of T cell development. CD3-related monoclonal antibodies (mAbs) are mainly used in the treatment of autoimmune diseases. Nearly half of all bispecific antibodies developed are used in tumor therapy, one of which is CD3 antigen. In this review, we discuss the importance of biological function and the crucial role of CD3 in tumor therapy. We highlight the research status of antibodies and small molecules targeting CD3 to provide guidance for future drug research.