Basically what it was saying that if a baby who has a nut allergy (for example) encounters nuts for the first time, they won't have a reaction, but their antibodies or whatever(they explained it better) will be reacting.
Then the second time the baby encounters nuts will be the time they react badly.
I thought this was interesting because I know a lot of parents (and have seen posts on here) who have watched their child closely the first time they give them nuts, then when they have been fine, presumed they are not allergic.
Hope this makes sense, not feeling very articulate tonight.