When the Immune System Overreacts

How can misguided immune responses cause problems?

The immune systems of some people overreact to harmless antigens, such as pollen, dust mites, mold, pet dander, and possibly their own cells. A strong immune response to harmless antigens can produce allergies, asthma, and autoimmune disease.

Allergies Antigens that cause allergic reactions are called allergens. When allergens enter the body of people affected by allergies, they trigger an inflammatory response by causing mast cells to release histamines. If this response occurs in the respiratory system, it increases mucus production and causes sneezing, watery eyes, a runny nose, and other irritations. Drugs called antihistamines help relieve allergy symptoms by counteracting the effects of histamines.

Asthma Allergic reactions in the respiratory system can create a dangerous condition called asthma. Asthma is a chronic disease in which air passages narrow, causing wheezing, coughing, and difficulty breathing. Both hereditary and environmental factors influence asthma symptoms. Asthma attacks can be triggered by respiratory infections, exercise, emotional stress, and certain medications. Other triggers include cold or dry air, pollen, dust, tobacco smoke, pollution, molds, and pet dander.

Asthma is serious and can be life-threatening. If treatment is not started early enough or if medications are not taken properly, severe asthma can lead to permanent damage or destruction of lung tissue. There is no cure, but people with asthma can sometimes control the condition. If the attacks are caused by an allergen, tests can identify which allergens cause the problem. Inhaled medications can relax smooth muscles around the airways and relieve asthma symptoms.

In Your Notebook Sometimes allergies are described as “overreactions of the immune system.” Explain what that phrase means.

FIGURE 35–17 Allergens Pet dander, dead skin shed from cats and dogs, is a common allergen (SEM 40X).