Experimental animal studies indicate that under normal steady-state conditions, initial encounters with non-pathogenic antigens (i.e. inert proteins, such as pollens) do not trigger immune responses, but instead activate antigen-specific suppressor T cells which protect against subsequent allergic sensitization by inducing a state of immunological tolerance towards the inhaled antigen. This inhalation tolerance phenomenon appears analogous to the more familiar process of oral tolerance in the gastrointestinal tract, and is proposed to serve as the ‘last line of defence’ against sensitization to aero-allergens which have evaded other levels of containment in the respiratory tract. It is postulated that this tolerance process plays a similar protective role in man, as the final component of a multi-layer defence system centering upon the respiratory epithelium. Recent findings which indicate that both oral and inhalation tolerance mechanisms are relatively slow to develop postnatally, provides a possible basis for the increased risk of allergic sensitization recognized in infancy.