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.