Won't repeat what has already been eloquently said....that's why
respiratory and digestive disease are most dramatically "reduced" in
breastfed babies (or "increased" in formula fed babies, to be truly
correct)--the SIgA protects babies directly in the gut and respiratory
tract so they don't have to be absorbed.  Same with the white blood cells
in the milk, for the most part, although they may be able to cross the
mucosa.  The other antibodies which may be present in the milk would be
treated just like other proteins....but the newborn gut is pretty
immature and even fairly large molecules can get through the mucosa.
That's why cow's milk protein and other allergens can cause
problems in young infants even though they may be digested more thoroughly
and be less of a problem in the more mature gut.

--
Anne Montgomery, M.D., I.B.C.L.C.
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St.Peter Hospital Family Practice Residency