Common allergenic pollens, fungi, animals, and arthropods.

Certain plants produce prodigious amounts of pollen. A single plant can produce 1 million to several million pollen grains in a day. In general, brightly colored flowering plants are of little clinical importance in inhalant allergy because their pollen is designed to be carried by insects (entomophilous) rather than by the wind (anemophilous). Many grasses, trees and weeds produce sufficient quantities of lightweight pollen to sensitize genetically susceptible individuals. The pollinating season of the various plants depends on the individual species and on the geographical location. However, for any particular plant in a given locale, the pollinating season is constant from year to year. Weather conditions, such as temperature and rainfall, influence the amount of pollen produced, but not the onset and termination of the pollen season. Pollen sampling and morphological identification can be routinely performed to determine the plant source and allergen load. Diagnosis of seasonal rhinitis is partly dependent on the correlation between the onset of symptoms and plant pollination time. Approx 60– 75% of seasonal rhinitis patients have positive skin test reactions to weed pollens, 40% to grass pollens, and 10% to tree pollens. Common plants causing allergy in humans are presented in Table 1. The listing is abbreviated and the reader should consult the tables in the article by Morrow and Slater in this volume for a more extensive listing of allergenic plants. The general category of “weeds” is used to group those invasive, undesirable plants that are not trees or grasses.