Atopic versus infectious diseases in childhood: a question of balance?

There is an increasing interest in the concept that respiratory tract infections during early childhood may in some circumstances confer protection against sensitisation to aeroallergens, via ''bystander'' stimulation of Th-l associated immune functions in the regional lymph nodes draining the airway mucosa. We hypothesise below that this phenomenon may be but one component of a broader process operative during early postnatal life, in which generalised contact with the microbial environment plays an obligatory role in stimulating the functional maturation of the Th-l arm of the immune response. We argue further that one of the most potent sources of such stimulation is provided by the normal commensal flora of the gastrointestinal tract, which is establish during early infancy.