From an equine (horse) reference book:

"To be effective in protection against disease, however, colostrum must be ingested within a few hours after birth, preferably within 15 to 30 minutes, because gut closure occurs about 24 to 30 hours after birth.  Subsequently, the foal digests these large molecular weight proteins, with the loss of their immunization properties."  In other words, the immune factors are then not able to cross directly through the gut and into general circulation.  Rather, they are mostly destroyed during the digestive process.

Another book says that there is a finite amount of colostrum produced for the first two days after birth, with the quick emergence thereafter of the mature milk.  You should also know that many animals, such as horses and cows, do not  get any immunity before birth (antibodies do not cross the placenta into the fetus), so that everything is in those few colostrum meals soon after birth.  If these babies do not receive colostrum, they will usually die fairly soon of one type of infection or another.

Human babies, on the other hand, do receive cross-placental immunities as well as a large dose of immunoglobulins from the colostrum and also continue to receive immune substances from the mature milk as well.  There is no "window" in the human baby which closes rapidly after birth.  Rather, the gut matures very slowly ( hence the admonition to allergy-prone moms to exclusive bf for as long as possible) and immunities are received for as long as the baby (or child) continues to nurse.

This is probably not as clear as it should be.  I have been up for more than 24 hours and need to go to sleep, but could not resist one quick peek at Lactnet before hitting the pillow.  I know there are a couple of horse breeders and cow farmers out there that can explain this far better than I.  My daughter has all of our best reference books at college with here (she is majoring in equine management.)


Warmly,
Sharon Knorr, BSMT, ASCP, IBCLC
Newark, NY (near Rochester on Lake Ontario)
mailto:[log in to unmask]

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