Something in the Air: Airborne Allergens

Introduction

Sneezing is not always the symptom of a cold. Sometimes, it is an allergic reaction to something in the air. Experts estimate that 35 million Americans suffer from upper respiratory symptoms that are allergic reactions to airborne pollen. Pollen allergy, commonly called hay fever, is one of the most common chronic diseases in the United States. Worldwide, airborne dust causes the most problems for people with allergies. The respiratory symptoms of asthma, which affects approximately 15 million Americans, are often provoked by airborne allergens (substances that cause an allergic reaction).

Overall, allergic diseases are among the major causes of illness and disability in the United States, affecting as many as 40 to 50 million Americans. The National Institute of Allergy and Infectious Diseases, a component of the National Institutes of Health, conducts and supports research on allergic diseases. The goals of this research are to provide a better understanding of the causes of allergy, to improve the methods for diagnosing and treating allergic reactions, and eventually to prevent allergies. This booklet summarizes what is known about the causes and symptoms of allergic reactions to airborne allergens, how these reactions are diagnosed and treated, and what medical researchers are doing to help people who suffer from these allergies.

What is an allergy?

An allergy is a specific immunologic reaction to a normally harmless substance, one that does not bother most people. People who have allergies often are sensitive to more than one substance. Types of allergens that cause allergic reactions include pollens, dust particles, mold spores, food, latex rubber, insect venom, or medicines.

Why are some people allergic to these substances while others are not?

Scientists think that people inherit a tendency to be allergic, meaning an increased likelihood of being allergic to one or more allergens, although they probably do not have an inherited tendency to be allergic to any specific allergens. Children are much more likely to develop allergies if their parents have allergies, even if only one parent is allergic. Exposure to allergens at certain times when the body's defenses are lowered or weakened, such as after a viral infection or during pregnancy, seems to contribute to the development of allergies.

What is an allergic reaction?

Normally, the immune system functions as the body's defense against invading agents such as bacteria and viruses. In most allergic reactions, however, the immune system is responding to a false alarm. When an allergic person first comes into contact with an allergen, the immune system treats the allergen as an invader and mobilizes to attack. The immune system does this by generating large amounts of a type of antibody (a disease-fighting protein) called immunoglobin E, or IgE. Each IgE antibody is specific for one particular allergenic (allergy-producing) substance. In the case of pollen allergy, the antibody is specific for each type of pollen: one type of antibody may be produced to react against oak pollen and another against ragweed pollen, for example.

These IgE molecules are special because IgE is the only class of antibody that attaches tightly to the body's mast cells, which are tissue cells, and to basophils, which are blood cells. When the allergen next encounters its specific IgE, it attaches to the antibody like a key fitting into a lock, signaling the cell to which the IgE is attached to release (and in some cases to produce) powerful inflammatory chemicals like histamine, cytokines, and leukotrienes. These chemicals act on tissues in various parts of the body, such as the respiratory system, and cause the symptoms of allergy.

Some people with allergy develop asthma. The symptoms of asthma include coughing, wheezing, and shortness of breath due to a narrowing of the bronchial passages (airways) in the lungs, and to excess mucus production and inflammation. Asthma can be disabling and sometimes can be fatal. If wheezing and shortness of breath accompany allergy symptoms, it is a signal that the bronchial tubes also have become involved, indicating the need for medical attention.