IgG4

IgG4 is the least abundant subclass of IgG in human serum and has unique functional features.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10123589/

IgG4 is largely unable to activate antibody-dependent immune effector responses and, furthermore, undergoes Fab (fragment antigen binding)-arm exchange, rendering it bispecific for antigen binding and functionally monovalent. These properties of IgG4 have a blocking effect, either on the immune response or on the target protein of IgG4.

The unique properties of IgG4 and its roles in health and disease

An immune evasion mechanism with IgG4 playing an essential role in cancer and implication for immunotherapy
https://jitc.bmj.com/content/8/2/e000661

13 August 2023
Class switch towards spike protein-specific IgG4 antibodies after SARS-CoV-2 mRNA vaccination depends on prior infection history
https://www.nature.com/articles/s41598-023-40103-x