Single-chain variable fragment

A single-chain variable fragment (scFv) is not actually a fragment of an antibody, but instead is a fusion protein of the variable regions of the heavy (V_H) and light chains (V_L) of immunoglobulins, connected with a short linker peptide of ten to about 25 amino acids.^[1] The linker is usually rich in glycine for flexibility, as well as serine or threonine for solubility, and can either connect the N-terminus of the V_H with the C-terminus of the V_L, or vice versa.^[2] This protein retains the specificity of the original immunoglobulin, despite removal of the constant regions and the introduction of the linker.^[3] The image to the right shows how this modification usually leaves the specificity unaltered.

These molecules were created to facilitate phage display, where it is highly convenient to express the antigen-binding domain as a single peptide. As an alternative, scFv can be created directly from subcloned heavy and light chains derived from a hybridoma. ScFvs have many uses, e.g., flow cytometry, immunohistochemistry, and as antigen-binding domains of artificial T cell receptors (chimeric antigen receptor).

Unlike monoclonal antibodies, which are often produced in mammalian cell cultures, scFvs are more often produced in bacteria cell cultures such as E. coli.^[3]