Allergies, which affect an estimated 50 million Americans, are the immune system's reaction to a normally harmless substance, such as dust, medication, certain foods, or pollen. It is not clear why one person will develop allergies and another will not, although the tendency may be inherited. Seasonal allergies occur at specific times of the year, while perennial allergies occur year-round. In many cases, allergies can be treated with medication, reducing contact with allergens, or shots.
(For more information, click Allergies. Topics discussed in this overview article include common triggers, other likely symptoms, a discussion on how the immune system responds to allergens, and more.)