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The intact antibody of human immunoglobulin （IgG） is composed of the fragment for antigen binding （Fab） and the crystallizable fragment （Fc） for binding of Fcγ receptors. Among the four subclasses of human IgG （IgG1, IgG2, IgG3, IgG4）, which differ in their constant regions, particularly in their hinges and CH2 domains, IgG1 has the highest FcγR-binding affinity, followed by IgG3, IgG2, and IgG4.