IgG4 is the predominant kind of antibody engaged in Fab-arm exchange. Human immunoglobulin G (IgG) subtype IgG4 makes up 3%–6% of the total IgG in human serum. Because IgG4 possesses a reversible reduction site (S228P) at the inter-heavy chain disulfide bond, which temporarily reduces the inter-heavy chain disulfide bond to half-antibodies, it varies from other IgG subtypes. IgG4 half-antibodies can recombine to create whole antibodies, which can lead to distinct light chain pairing or bispecificity. We refer to this phenomena as heavy chain exchange, or Fab-arm exchange.
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