Final answer:
The IgG antibody is the most abundant in blood and the only one able to cross the placenta, providing passive immunity to the fetus. It is essential for the body's defense against pathogens and activates the immune responses during reexposure to a pathogen or in reaction to a vaccine.
Step-by-step explanation:
The antibody that can cross the placenta and is the most abundant in blood is IgG. This antibody is a monomer, making up about 80% of the total serum antibody. IgG is not only the most abundant but also the most versatile class of antibody, playing a crucial role in the body's defense mechanisms against pathogens. It is efficient in penetrating tissue spaces, and uniquely capable of crossing the placental barrier to provide the developing fetus with passive immunity during pregnancy.
IgG antibodies are a major component of late primary immune responses and are the main antibodies of secondary immune responses due to class switching. They help to clear pathogens from the blood and can activate complement proteins, although not as effectively as IgM.
Natural passive immunity is achieved through the transfer of IgG from mother to child, which can occur during pregnancy across the placenta and after birth through breast milk, which contains secretory IgA. The transfer of IgG provides important passive immune defense to the infant that can last up to six months after birth.