A state of hypersensitivity induced by exposure to a particular antigen (allergen) resulting in harmful immunologic reactions on subsequent exposures, the term is usually used to refer to hypersensitivity to an environmental antigen (atopic allergy or contact dermatitis) or to drug allergy.
most common symptoms include:

• Runny nose
• Stuffy nose
• Sneezing
• Nasal itching (rubbing)
• Itchy ears and throat
• Post nasal drip (throat clearing)