Some years back at the ILCA conference, someone spoke on the presence of
"interleukins" in breastmilk.  Here's my understanding of what was said:

Interleukins are antiinfectives, but they're fragile and break down almost
immediately in the stomach.  Once they were discovered
in breastmilk, the initial feeling was that they were "just there" - that
they couldn't do the baby much good because they just didn't persist long
enough in the GI tract.

But interleukins are particularly effective against such viruses as
rhinosyncytial virus (RSV), which is harbored in a baby's upper
respiratory tract.  When a baby nurses, his pharynx is bathed in milk, he
gets milk up his nose... and he bathes his upper respiratory tract in...
interleukins!  It doesn't matter if they're destroyed once they hit his
stomach.  They've already acted to protect him against some particularly
nasty bugs.

The baby who gets Pedialyte when he has a cold misses, along with
everything else, that many antiviral baths!

Pretty neat, and I don't even know what "interleukins" are.

Diane Wiessinger, MS, IBCLC, LLLL  Ithaca, NY