Final answer:
In the hierarchy of complement activation efficiency among IgG subclasses, IgG3 and IgG1 are the most efficient, followed by IgG2, with IgG4 having minimal or no complement activating capacity. IgM is also highly efficient in activating complement but is not an IgG subclass.
Step-by-step explanation:
In terms of complement activation, the IgG subclasses activate complement with differing efficiencies. IgG1 and IgG3 are generally considered the most efficient in activating the complement pathway, particularly the classical pathway, due to their capability to bind C1q tightly. Following these, IgG2 is somewhat less effective, and IgG4 has very little to no ability to activate complement. IgM, although not an IgG subclass, is also very efficient in complement activation due to its pentameric structure, allowing for multiple antigen-binding sites that can lead to a strong and rapid activation of the complement system.
IgG antibodies play a crucial role in the body's defense against pathogens, having the ability to clear pathogens from the blood, activate complement proteins, and protectextracellular spaces by exiting the blood into the interstitial fluid. Furthermore, these antibodies are able to cross the placenta to protect the developing fetus from disease.