Final answer:
The IgG subclass IgG3 is most efficient at activating complement due to high affinity for complement proteins, while IgG4 is the least efficient because of its lower affinity.
Step-by-step explanation:
The IgG subclass IgG3 is most efficient at activating complement due to its higher affinity for complement proteins. The constant (Fc) region of the IgG3 antibody determines its class and activates the complement cascade, leading to a potent immune response. On the other hand, IgG4 is the least efficient at complement activation because it has a lower affinity for complement proteins, meaning it's less effective in eliciting this particular immune mechanism. IgG antibodies are critical components of the immune system, with class switching playing a major role in their response to pathogens.