An infant is given formula containing large intact antibodies. Which intestinal uptake mechanism most likely explains how these macromolecules can cross neonatal enterocytes intact, and why does this process decline with age?

• A. Endocytosis (including receptor-mediated transcytosis) allows enterocytes to engulf and transport intact antibodies; it declines as the gut matures and tight junctions and proteolytic activity increase.
• B. Direct secretion from the liver into the neonatal bloodstream bypassing the intestine entirely; intestinal uptake does not explain neonatal antibody presence.
• C. Passive diffusion through the lipid bilayer due to antibodies becoming hydrophobic in the neonatal gut, and this hydrophobicity is lost later in life.
• D. Facilitated diffusion through general ion channels because antibodies mimic electrolytes in neonates, and channel expression increases with age causing decreased antibody proteostasis.

Answer: (A)