Pollen: Small, light, dry protein particles from trees, grasses, flowers, and weeds that may be spread by the wind. Pollen particles are usually the male sex cells of a plant, and they are smaller than the tip of a pin. Pollen is a potent stimulator of allergic responses. It lodges in the mucous membranes that line the nose and in other parts of the respiratory tract, causing irritation and histamine reactions.
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