
I've seen children do this and wondered about something.  If the
mucus in one's nose collects (filters) particles going into the
airway, if a child then picks and ingests this material, might
it have a vaccinatory effect, since if the body ingests airborne
diseases or other 'stuff' on the mucus, the body might generate
antibodies for this small "invasion"?

Maybe this is why some children don't get sick very often? :-)
                         - tom