Deposition in the respiratory system of an inhaled dry particle depends on many factors, particularly its aerodynamic diameter. The quantity deposited in each compartment of the respiratory tract can be approximately determined by mathematical laws, but ther are still controversies about large size particles. Thus pollens have been found by the authors beyond the larynx deep in the lung. The bronchopulmonary system is divided in two parts, anatomically and immunologically different. The local response, closely related to the antigen, depends also on the intimate contact between particles and the immunity defense system of the individual. Liberation and solubility of some antigenic fractions, perhaps enzymatic, would help their intra-epithelial penetration and contact with the secretory immunity system. The additional intervention of an extraepithelial response has been observed in bronchi after instillation of pollen in the rat. There is in the sensitized animal an intense cellular reaction amidst the mucous of bronchial lumen with macrophages adhering to pollen granules and forming rosettes.