Originally Posted by bluets
I learned from a researcher who studies comparative neonatal gut development that human infants have a permeable gut for only a few days - permeable to small water-soluble substances (it closes up on its own). Otherwise, the gut is only permeable if damaged. In other animals, the neonatal gut is wide open, allowing take-up of IgA from mama; in humans, babies get their passive immune protection in the form of IgG prenatally through the placenta. However, small quantities of big antigens may get across the human neonatal gut - this may be a good thing as it "tolerises" the infant immune system and makes it capable of recognizing and dealing with appropriately with them later in life. The baby's gut also has cells on its surface (in Peyer's patches - lymphoid tissue) that recognize and deal with "foreign" antigens - whether or not they traverse the gut barrier. Finally, in mastitis, big molecules (both antigens and antibodies) can "leak" into milk.