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.
Although the exact genetic factors are not yet understood, the tendency to allergies, as well as to allergic disease, is linked to heredity.
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.
The problem is an oversensitivity of the lungs and airways, which overreact to certain "triggers" and become inflamed and clogged.
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.