The pathophysiological mechanisms of pollen-induced asthma have until now remained unclear, because pollen particles have generally been considered too large to penetrate into the lower respiratory tract. Such grains are therefore believed to be unable to induce an immunological response in the lower respiratory tract. There is evidence, however, that a small percentage of large particles (20-30 microns) may penetrate into the peripheral areas of the lung. It also appears that small airborne units of less than 5 microns with the same antigenic activity as pollen, may contribute to inhaled allergen burdens. Pollen asthma probably results from a gradual cumulative effect of deposition of small amounts of allergen in lower airways, and is hence poorly correlated with daily pollen counts.