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Allergy & its mechanism
Types of allergic reactions
Airborne allergens
Dust mites
Mould
Pollens
Cockroach
Animal dander
Smoke and combustion gas
Toner dust
VOCs emissions
Particulate Pollutants
Particulate Pollutants
Food allergens & modified food
Contact & proximity allergens
Allergens from insect stings
 
Pollens
Magnified grass pollen
  The major groups of pollens that cause allergic reactions (called hay fever) are the tree, grass and weed pollens. These pollens are relatively lighter in weight than the pollens of colourful flowers, hence easily dispersed by wind. Trees generally produce pollen in the spring, grasses in the early summer and most ragweed species in late summer.
  Each plant has a pollinating period that is more or less the same form year to year. The exactly time of pollination depends on the relative length of day and night, and therefore on geographic location. The farther north you go in the northern hemisphere, the later the pollinating period and the allergy season.
  The weather conditions can affect the amount of pollens produced and distributed. On a 24 hours basis, pollen counts tend to be highest early in the morning on warm, dry, breezy days, and lowest during chilly, wet periods.
  Since airborne pollens can be carried for long distances, getting rid of an offending plant in an area usually would not help much. In cities, the heat generated by urban activities keeps pollens airborne, so that ground-level measures of pollens are lower than in the country. However, interaction of the pollens with city pollution leads to allergic symptoms occurring at relatively low allergen levels.
  Ways to reduce exposure to pollens:
  If possible, limit your time outside during the pollen season, especially in the early morning and late evening when pollen levels often peak.
  Keep windows at home and in the car closed when not in use.
  Don't hang your clothes outside to dry if there is a risk of pollen contamination.
  Avoid buying trees that produce pollens likely to aggravate allergies for your garden or as indoor plants.
  Pollens can be transported via people and pets. Avoid keeping domestic pets.
 
Examples of plants that produce allergy-causing pollens
Trees:

Elms, maples, ashes, oaks, walnuts, hickories, sycamores, mulberries, pecan, box elder and mountain cedar.
Grasses: Bermuda, Johnson, timothy, redtop, sweet vernal and orchard grasses.
Weeds: Ragweed (in the US), Russian thistles, sages, and amaranths.
 
Allergy to pollens from colourful flowers
For colourful and scented flowers like roses, usually only florists, gardeners and others who have prolonged, close contact with flowers are likely to become sensitized to pollen from these flowers. The pollens of these colourful and scented flowers are mainly carried by insects such as butterflies and bees instead of by wind.