Causes

The allergic process, called atopy, occurs when the body overreacts to a substance that it senses as a foreign “invader." The immune system works continuously to protect the body from potentially dangerous intruders such as bacteria, viruses, and toxins. However, for reasons not completely understood, some people are hypersensitive to substances that are typically harmless. When the immune system inaccurately identifies these substances (allergens) as harmful, an allergic reaction and inflammatory response occurs.

• The antibody immunoglobulin E (IgE) is a key player in allergic reactions. When an allergen enters the body, the immune system produces IgE antibodies. These antibodies then attach themselves to mast cells, which are found in the nose, eyes, lungs, and digestive tract.
• The mast cells release inflammatory chemical mediators, such as histamine, that cause atopic symptoms (sneezing, coughing, wheezing). The mast cells continue to produce more inflammatory chemicals that stimulate the production of more IgE, continuing the allergic process.

There are many types of IgE antibodies, and each are associated with a specific allergen. This is why some people are allergic to cat dander, while others are not bothered by cats yet are allergic to pollen. In allergic rhinitis, the allergic reaction begins when an allergen comes into contact with the mucous membranes in the lining of the nose.

Triggers of Seasonal Allergic Rhinitis (Hay Fever)

Seasonal allergic rhinitis occurs only during periods of intense airborne pollen or spores. It is commonly, although inaccurately, called hay fever. No fever accompanies this condition, and the allergic response is not dependent on hay. In general, triggers of seasonal allergy in the U.S. include:

• Ragweed. Ragweed is the most dominant cause of allergic rhinitis in the U.S., affecting about 75% of allergy sufferers. One plant can release 1 million pollen grains a day. Ragweed occurs everywhere in the U.S., although it is less common in western coastal states, southern Florida, northern Maine, Alaska, and Hawaii. The effects of ragweed in the northern states are first felt in middle to late August and last until the first frost. Ragweed allergies tend to be most severe before midday.
• Grasses. Grasses affect people in mid-May to late June. Grass allergies are experienced more in the late afternoon.
• Tree Pollen. Small pollen grains from certain trees usually produce symptoms in late March and early April.
• Mold Spores. Mold spores that grow on dead leaves and release spores into the air are common allergens throughout the spring, summer and fall. Mold spores may peak on dry windy afternoons or on damp or rainy days in the early morning.

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Review Date: 05/03/2011
Reviewed By: Harvey Simon, MD, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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