The epipthelial surfaces of the gut and respiratory tract provide a mechanical barrier to invasion by microorganisms. There is a constant flow of mucus in the respiratory tract that is moved along by the action of the ciliated columnar epithelium. These layers also secrete a variety of chemicals that are useful in preventing invasion.
It is a bit confusing to cover the likes of hydrochloric acid in the barrier function, one may feel that it might be best discussed in humoral components. This division is artificial and while hydrochloric acid is included in barriers, lysozyme and complement are included in humoral. The distinction appears to be that if a molecule has a specific immunological function then it is termed a humoral component, however, if it acts to benefit the immune respones in a general fashion (ie general toxin) then it is part of the barrier.