ScFv kda is an antibody made up of short peptides of 15-20 amino acids linked to the variable regions of heavy chain and light chain of antibody.ScFv can retain its affinity to antigen, and has the characteristics of small molecular weight, strong penetration and weak antigenicity. It is a synthetic antibody expressed in E. coli by genetic engineering technology. There is only one chain of the complete antibody, so it is called single chain antibody. There are three forms of scFv: direct expression in cytoplasm, fusion with other proteins, and secretion and expression of functional scFv.An alternative approach is to split the dimeric variable domains from normal IgG of humans or mice into monomers by camel zing a few critical residues.

In recent years, great progress has been made in the fields of biology, medicine, laboratory research and diagnosis and treatment of diseases. However, single-chain antibodies often have some disadvantages, such as low affinity, single function, poor stability and rapid elimination in vivo, which restrict their wide application. Therefore, on the basis of SCFv, ScFv polymers with good binding properties, such as Diabodies, Tribodies and T7 phage library, have been developed in recent years. The conversion between these polymers only depends on the change of Linker length. When the amino acids are reduced from 12 to 0, the diabodies are converted to Tribodies and Tetribodies. These multi-valent, multi-potency and high affinity small molecule antibodies have many advantages over single chain antibodies, and their preparation and application have been studied in depth.ScFv's Linker is generally 15 amino acids. Glycine and serine are often used to form a polypeptide Linker with certain elasticity and protease resistance. Its sequence is (GGGGS) Linker's function is not only to connect VH and VL, but also to maintain certain elasticity, so that VH and VL functional areas can still be paired after folding, forming a monovalent antigen complex. Loci. When Linker is shortened to 3-12 amino acids, the VH and VL regions of the same ScFv molecule cannot pair with each other, and the VH and VL functional regions from different molecules are paired into a bivalent dimer, namely, Diabodies; when Linker is further shortened to 0-2 amino acids, the C-terminal residues of peptide library are directly linked to the N-terminal residues of the VL, first by two. Each ScFv molecule forms a Diabodies, and the free VH and VL at both ends form a trivalent trimer with the third ScFv molecule, namely Tribodies.

The advantages of scFv in medicine are as follows: 1. It can remove non-specific competitive surface proteins, and the background of tumor imaging is clearer. 2. increase the concentration of drug treatment in infiltrating tumor tissue. 3. immunization is small, which can eliminate the rejection of human mice. 4. The half-life of circulation in the body is short, easy to clear, conducive to detoxification and discharge; 5. Easy to connect with toxins or enzyme genes, easy to obtain immunotoxins or enzyme-labeled antibodies directly. On the other hand, the disadvantages are low stability, single function and low affinity. Bispecific Diabodies, with their dual antigen binding specificity, have become a new component of immunodiagnosis and immunotherapy, and have broad application prospects. It is a diabodies composed of two different ScFv molecules. It is bi-specific and can bind to two antigens simultaneously. Compared with previous bispecific antibodies, bispecific diabodies have the following advantages: firstly, the deletion of Fc region reduces the chance of nonspecific binding with FcR positive cells; secondly, because of its small size, the host's anti-heterologous antibody response is minimized; Fourthly, it is easy to prepare and can bypass the time-consuming and laborious project of crabbit antibody library without chemical crosslinking agent, thus avoiding the potential immunogenicity and teratogenicity, and laying a foundation for the development of human bi-specific antibodies. In mediating the cytotoxicity of immune cells, bispecific diabodies can double recognize the specific antigens on tumor target cells and immune effector cells, concentrate effector cells on tumor sites, initiate signal transduction, and activate effector cells to play an anti-tumor role. Cytotoxicity mediated by bispecific diabodies includes monocytes, macrophages, natural killer cells and lymphocytes. The dual specificity of Diabodies is a good material for designing immunodiagnostic reagents. Kontemann et al. designed several diabodies, one arm was anti-E. coli galactosidase, the other arm was anti-egg white lysozyme (HEL), anti-CEA or anti-HIV-1 gp120. With these diabodies as maller outer capsid, HEL was detected by ELISA, CEA by immunohistochemistry, and gp120 by immunoblotting. The results were all detectable. The corresponding antigen indicates that bispecific Diabodies can be used in enzyme immunoassay.