(IgG1, IgG2, IgG3, and IgG4) are named in order of their relative serum concentrations
| Species | Isotype | ADCC | C' | FcγRI Act | FcγRIIB Inh | FcγRIII | FcγRIIIA Act | FcRn Trans |
|---|---|---|---|---|---|---|---|---|
| Human | IgG1 | ++++ | +++ | +++ | ++ | NA | ++ | +++ |
| Human | IgG2 | +/− | + | − | + | NA | + | +++ |
| Human | IgG3 | +++ | ++++ | +++ | ++ | NA | ++ | +++ |
| Human | IgG4 | + | +/− | +++ | ++ | NA | ++ | +++ |
| Species | Isotype | ADCC | C' | FcγRI Act | FcγRIIB Inh | FcγRIII | FcγRIIIA Act | FcRn Trans |
|---|---|---|---|---|---|---|---|---|
| Mouse | IgG1 | ++ | +/− | − | ++ | ++ | NA | +++ |
| Mouse | IgG2a | +++ | ++++ | +++ | ++ | ++ | NA | +++ |
| Mouse | IgG2b | ++++ | +++ | ++ | ++ | ++ | NA | +++ |
| Mouse | IgG3 | +/− | ++ | + | − | − | NA | +++ |
IgG antibodies are the hallmark of immunological memory in the humoral immune response. In addition to the isotype switch from IgM to IgG in a secondary antibody response, somatic hypermutation can lead to affinity maturation, a process by which the average affinity of antibody for the antigen eliciting the immune response can increase.
某些亚类的IgG抗体直接或通过激活携带补体或FcR的细胞来促进免疫。直接通过抗体结合介导的免疫的重要例子包括毒素（如白喉毒素）和病毒（如脊髓灰质炎病毒）的中和。IgG诱导的补体激活的医学上重要的例子包括对包囊细菌病原体的免疫，导致吞噬细胞内的调理和破坏（如肺炎链球菌）或直接补体介导的裂解（如脑膜炎奈瑟菌）。IgG抗体激活Fc受体细胞也与对病原体（如新生隐球菌）的免疫有关
The consensus view is that human IgG1 and IgG3 isotypes are effective activators of the classical complement pathway. Although some older sources state that IgG2 and IgG4 are weak or nonactivators of the classical complement pathway, more recent evidence suggests that, when epitope density is high, IgG2 is effective in activating complement through the classical pathway.
One possible source for the isotype-related variation in complement-activating ability is variation in affinity for C1q (IgG3>IgG1>IgG2>IgG4)
However, isotype-associated differences in complement activation have also been found to occur at steps of the cascade subsequent to the binding of C1q to antibody.
For example, in one study of chimeric monoclonal antibodies engineered to express identical V domains and representing all four human IgG subclasses, the IgG3 antibody fixed C1q better than the IgG1 antibody but the IgG1 molecule was more effective in mediating complement-dependent cell lysis than the IgG3 molecule. This may reflect the ability of hexameric assemblies of human IgG1, which are mediated by noncovalent Fc-Fc interactions, to maximally activate complement. Human IgG3 has been largely understudied. This reflects, in part, that the structure and enhanced functionality of human IgG3 is unique when compared with most mammals, including macaque(猕猴) species, which do not have an equivalent analog among their IgG subclasses. Prehuman clinical studies using macaque species can thus not be used to assess the human IgG3 response to vaccines.
A remarkable attribute of IgG (for three of the four subclasses) is its serum half-life of about 23 days.
This property, attributable to the Fc region and its interaction with the neonatal Fc receptor (FcRn), has been exploited for therapeutics through the genetic fusion of solubilized receptors (eg, CTLA4) to IgG Fc regions.