Subclasses
There are four IgG subclasses (IgG1, 2, 3, and 4) in humans, named in order of their abundance in serum (IgG1 being the most abundant).
| Name | Percent | Crosses placenta easily | Complement activator | Binds to Fc receptor on phagocytic cells |
| IgG1 | 66% | yes (1.47)† | second-highest | high affinity |
| IgG2 | 23% | no (0.8)† | third-highest | extremely low affinity |
| IgG3 | 7% | yes (1.17)† | highest | high affinity |
| IgG4 | 4% | yes (1.15)† | no | intermediate affinity |
| †: Quota cord/maternity concentrations blood. Based on data from a Japanese study on 228 mothers. | ||||
Note: IgG affinity to Fc receptors on phagocytic cells is specific to individual species from which the antibody comes as well as the class. The structure of the hinge regions gives each of the 4 IgG classes its unique biological profile. Even though there is about 95% similarity between their Fc regions, the structure of the hinge regions is relatively different.
In a model of autoantibody mediated anemia using IgG isotype switch variants of an anti erythrocytes autoantibody, it was found that IgG2a was superior to IgG1 in activating complement. Moreover, it was found that the IgG2a isotype was able to interact very efficiently with FcgammaR. As a result, 20 times higher doses of IgG1 in relationship to IgG2a autoantibodies, were requried to induce autoantibody mediated pathology.
Azeredo et al. J of Exp Med. 2002. 195: 665.
Read more about this topic: Immunoglobulin G