Allergies are an overreaction of the immune system to certain stimuli.
One view is that allergies can develop as the result of childhood exposure
to certain irritants, such as dust or animal dander, while the immune
system is not yet mature. Another view is that exposure to particular
bacteria early in childhood actually triggers the proper development of
the immune system, and that limiting exposure to these bacteria through
excessive hygiene can cause children's immune systems to overreact to
certain irritants later on. A new study supports the second view: children
who are washed especially frequently and whose parents clean their homes
especially frequently are more likely to develop allergies than are other
children. So in order to reduce the incidence of allergies in children and
adults, parents should not limit children's exposure to irritants or
bacteria.