An allergy is the body's immune system reacting to an outside substance or agent that is normally harmless to others. This reaction manifests in the form of coughing, wheezing and sneezing, and sometimes asthma. In extreme cases, an allergic reaction can lead to anaphylaxis, during which an individual can experience confusion, abdominal pain, heart palpitations, dizziness and shortness of breath, among others. Types of allergic "triggers" include indoor and outdoor allergies (or seasonal allergies), eye allergies, latex allergies, food/drug allergies and insect allergies.
From a Chinese Medicine perspective, allergies often involve the "Lung" system in Chinese Medicine terms. For general information on what is meant by the "lungs" in Chinese Medicine, you can read "My Lungs Are What?".