Recombinant Anti-CD28 x Anti-CD3 Bispecific Antibody (VHH-IgG) is formed by an IgG and two VHH fragments. The VHHs from an anti-CD3 antibody variable domain are respectively connected to the N terminus of the anti-CD28 IgG heavy chains or light chains to form VHH-(H)IgG or VHH-(L)IgG. It is a type of appended IgGs and has 2+2 antigen-binding valency. This BsAb can improve T-cell activation. It is designed for the research of Tumors therapy.