Ace Microbiology for COMLEX Level 1 2026 – Dive Into Discovery and Dominate Your Exam!

IgG is the antibody that fits both criteria. In adults, it is the most abundant immunoglobulin circulating in serum, and its relatively small monomeric size enables it to be transported across the placenta via the neonatal Fc receptor (FcRn). This placental transfer provides the fetus with passive immunity. In contrast, IgM is a pentamer and too large to cross the placenta efficiently, IgA is mainly secreted as a dimer in mucosal secretions with only smaller serum levels, and IgE is present in much lower amounts and functions primarily in allergies and antiparasitic defense.