What is an allergy
define allergy
Allergies are caused by the immune system overreacting to a normally harmless substance. This substance, called an allergen , comes in a multitude of forms, from the pollen of a certain type of tree to an ingredient in a medicine, the dander of a puppy to a food like peanuts. Not everyone has allergies. But among those who do suffer from it, some are allergic to one thing in particular, while others react in very different ways to many different allergens.
Seasonal outdoor allergies ( seasonal allergic rhinitis ) and indoor allergies that last all year round ( perennial allergic rhinitis ) are among the most common allergies. People who sneeze and snort only during certain times of the year owe their symptoms to pollen. Tree pollen proliferates in the spring, while grass and weed pollen takes over in late summer and early fall. Indoor allergens like dust, mold, and pet dander can irritate eyes, nose and throat year round.
