One of the marvels of the human body is that it can defend itself against harmful invaders such as viruses or bacteria. In some people, the body reacts to harmless substances such as dust, mold or pollen by producing an antibody called immunoglobulin E (IgE). When patients with one of the allergic diseases (such as rhinitis or asthma) are exposed to these substances, the immune system then rallies its defenses, launching a host of complex chemical weapons to attack and destroy the supposed enemy. In the process, some unpleasant and, in extreme cases, life-threatening symptoms may be experienced.
Hundreds or even thousands of ordinary substances can trigger allergic reactions. These are called "allergens." Among the most common are plant pollens, molds, household dust (dust mites), animal dander, industrial chemicals, foods, medicines and insect stings.

An allergic reaction may occur anywhere in the body, but usually appears in the skin, eyes, lining of the stomach, nose, sinuses, throat and lungs -- places where special immune system cells are stationed to fight off invaders that are inhaled, swallowed or come in contact with the skin. 

Allergies can affect anyone, regardless of age, gender, race or socioeconomic status. While it's true that allergies are more common in children, they can occur for the first time at any age or, in some cases, recur after many years of remission.

Although the exact genetic factors are not yet understood, the tendency to allergies, as well as to allergic disease, is linked to heredity. 

The best first step in the diagnosis of allergies is a thorough health history and physical examination. If you have allergy symptoms that occur in association with exposure to certain things, that is highly significant. Allergy diagnostic tests, such as skin tests or blood tests, provide similar information and merely confirm what your health history tells the doctor. If your doctor were to rely exclusively on the results of skin or blood tests (without history and physical examination), you could be diagnosed as having an allergic problem that you don't necessarily have.

Skin tests, in most situations, are preferable because (1) the results are available immediately, (2) they are less expensive and (3) they are more sensitive to subtle allergies.

A blood test is appropriate in certain situations, particularly when you (1) cannot suspend antihistamine therapy which can inhibit skin tests, (2) have widespread skin disease making skin testing difficult, (3) are so sensitive to the allergen that the test might be risky or (4) cannot be skin tested for some other reason.  

Asthma is a chronic, inflammatory lung disease characterized by recurrent breathing problems. People with asthma have acute episodes when the air passages in their lungs get narrower, and breathing becomes more difficult.

The problem is an oversensitivity of the lungs and airways, which overreact to certain "triggers" and become inflamed and clogged. 

The cause of the lung abnormality that is asthma is not yet known. Through research, scientists have established that the disease is a special type of inflammation of the airway that leads to contraction of airway muscle, mucus production and swelling in the airways. The airways become overly responsive to environmental changes. The result is wheezing and coughing. 
As yet there is no cure for asthma, but asthma can be controlled with proper treatment. People with asthma can use medicine prescribed by their physician to prevent or relieve their symptoms, and they can learn ways to manage episodes. They also can learn to identify and avoid the things that trigger an episode. By educating themselves about medications and other asthma management strategies, most people with asthma can gain control of the disease and live an active life. 
Asthma is sometimes hard to diagnose because it can resemble other respiratory problems such as emphysema, bronchitis and lower respiratory infections. For that reason, asthma is underdiagnosed – that is, many people with the disease do not know they have it and therefore are never treated. Sometimes the only symptom is a chronic cough, especially at night. Or, coughing or wheezing may occur only with exercise. Some people mistakenly think they are having recurrent bronchitis, since respiratory infections usually settle in the chest in a person predisposed to asthma.

To diagnose asthma and distinguish it from other lung disorders, physicians rely on a combination of medical history, a thorough physical examination, and certain laboratory tests. These tests include spirometry (using an instrument that measure the air taken into and out of the lungs), peak flow monitoring (another measure of lung function), chest X-rays and sometimes blood and allergy tests.