Final answer:
The efficiency order for placental transfer of IgG subclasses is IgG1, followed by IgG3, IgG4, and least efficiently, IgG2.
Step-by-step explanation:
The order of placental transfer efficiency for IgG subclasses from most to least efficient is IgG1 > IgG3 > IgG4 > IgG2. This transfer provides natural passive immunity to the developing fetus, a crucial aspect of the infant's immune defense, which can last up to six months after birth. IgG is the most versatile class of antibody and the only one able to cross the placenta, with IgG1 being the most efficient in doing so. IgG also plays a role in clearing pathogens from the blood, activating complement proteins, and fighting extracellular pathogens. Among the IgG subclasses, the efficiency of this process can vary, which is reflected in the given order.