Recombinant Anti-CD28 x Anti-CD3 Bispecific Antibody (VHH-Fc-VHH, Tetravalent HCAb) is a fusion protein of two diabodies connected together by an Fc region. In a diabody, the two VHHs with short linkers are from anti-CD28 and anti-CD3 antibodies. The VH and VL domains form an inter-chain pairing. The diabody with different specificities is fused to the Fc fragment, which results in 2+2 antigen-binding valency and exerts Fc-mediated functions or just extends the half-life. This BsAb can improve T-cell activation. It is designed for the research of Tumors therapy.