Cause Hay fever , also called allergic rhinitis , is an allergy to pollen or other airborne substances. Plants produce pollen in the spring. The pollens of some plants are very light and dry and carried in the air. These are the pollens that cause hay fever. The most common irritants are pollens of weeds such as ragweed, sagebrush, and redroot pigweed.
Many people are also allergic to grasses such as timothy, redtops, and rye.
Various common trees also produce airborne pollens, including elms, maples, oaks, and birches. Other causes of hay fever include floating bits of mold, dust, and dandruff from animals. Any substance that provokes an allergic reaction is called an allergen. If you have an allergy to a particular substance in the air, breathing it in will cause a reaction. In all allergies , your body treats the allergen as if it were a poison and produces antibodies to fight it.