Diabody is a noncovalent dimer of single-chain Fv (scFv) fragment that consists of the heavy chain variable (VH) and light chain variable (VL) regions connected by a small peptide linker. Another form of diabody is single-chain (Fv)2 in which two scFv fragments are covalently linked to each other. bispecific bivalent dimers are produced by using two different chains with the same orientation, the first containing the VH of Antibody 1 and the VL of Antibody 2, and the second containing the VH of Antibody 2 and the VL of Antibody 1 (Fig. 1). Varying the linker length by only one amino acid may shift the equilibrium completely to another oligomeric state with the lowest complexity being most favorable. Diabodies are one of the smallest recombinant BsAbs and the distance between the two antigen-binding sites is only 6.5 nm on average, which is less than half the distance in IgG. This compact size contribute to rapid pharmacokinetics, low immunogenicity and high tumor penetration.

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Fig. 1. A bispecific diabody composed of two different chains, each containing a VL and VH from different antibodies, in a head-to-tail arrangement (adopted from: Bates A, Power CA. (2019) David vs. Goliath: The Structure, Function, and Clinical Prospects of Antibody Fragments. Antibodies, 8:28). 

Formats of bispecific antibodies (BsAbs)

Many formats have been developed for BsAb generation as listed in the following table.

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