Allergic responses to inhaled antigens occur when an individual is first sensitized to the antigen (i. e., the allergen), inducing an immune response, and then has a subsequent exposure to the same antigen. The sensitization phase is characterized by:

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.