Final answer:
IgG is the most abundant antibody in human blood, making up about 80-85% of antibodies and efficiently entering tissues. It also crosses the placenta to provide immunity to fetuses.
Step-by-step explanation:
IgG is the predominant antibody in human blood, accounting for 80-85% of circulating antibody levels. It is a monomeric antibody that is not only important for fighting against diseases due to its specificity for antigens but also because it can easily leave the bloodstream and enter tissues, providing versatile defense mechanisms against pathogens. Additionally, IgG is unique in its ability to cross the placental barrier, thereby offering passive immunity to the developing fetus during pregnancy.