Increased circulating IgG concentration Increased circulating IgG subclass 2019-02-12T22:46:56Z The four subclasses, IgG1, IgG2, IgG3, and IgG4, which are highly conserved, differ in their constant region, particularly in their hinges and upper CH2 domains. These regions are involved in binding to both IgG-Fc receptors (Fc-gammaR) and C1q. As a result, the different subclasses have different effector functions, An elevation of circulating IgG level predominantly related to an elevation of one of the four IgG subclasses.