An allergen is something that triggers an allergy. When a person with allergic rhinitis breathes in an allergen such as pollen, mold, animal dander, or dust, the body releases chemicals that cause allergy symptoms.
Plants that cause hay fever are trees, grasses, and weeds. Their pollen is carried by the wind. (Flower pollen is carried by insects and does not cause hay fever.) Types of plants that cause hay fever vary from person to person and from area to area.
The amount of pollen in the air can affect whether hay fever symptoms develop or not.
Hot, dry, windy days are more likely to have a lot of pollen in the air.
On cool, damp, rainy days, most pollen is washed to the ground.
Hay fever and allergies often run in families. If both of your parents have hay fever or other allergies, you are likely to have hay fever and allergies, too. The chance is higher if your mother has allergies.