When an allergen such as pollen enters the body of a person with a sensitized immune system, it triggers antibody production. Histamine and other chemicals are released by body tissues as part of the immune response. This causes itching , swelling of affected tissues, mucus production, muscle spasms , and other symptoms. Many disorders are associated with, triggered, or worsened by allergies. These include eczema , asthma , and many others.
Allergies are very common.
There is no known way to prevent development of hay fever. Symptoms may sometimes be prevented by avoiding known allergens. Most trees produce pollen in the spring, grasses and flowers usually produce pollen during the summer, and ragweed and other late-blooming plants produce pollen during late summer and early autumn. History is important in diagnosing allergies , including whether the symptoms vary according to time of day or the season and possible exposures. Complement levels may be abnormal.