Allergy is a hypersensitivity disorder that includes the immune system reactions to things that are typically harmless to most people.  Allergic reactions occur when your immune system mistakenly believes that certain substance is harmful to your body.  Substances that cause allergic reaction includes certain foods, dust, plants, pollen or medicines, and are called allergens.  The immune system produces antibodies to that allergen in an attempt to protect our body.  Those antibodies cause certain cells to produce chemical in the bloodstream and one the chemical released is called histamine.  The histamine reacts on the nose causing runny nose, on the eyes making it red or puffy, on the skin manifested by rashes, on throat and lungs causing asthma attacks.  If exposed to the same allergen again, the same reaction of the antibody will occur, meaning, that every time you come into contact with tat allergen, you will have an allergic reaction.  Allergic reaction can be mild, but in some people severe allergies may result in life threatening reactions called anaphylaxis.  Anaphylaxis occurs usually minutes after exposure to the allergen but some reaction might be delayed by few hours.  Signs of anaphylaxis include swelling lips, tongue, throat or other body parts, difficulty in breathing and swallowing, and dizziness or loss of consciousness.  Allergy remedies include the use of medications like anti histamines that specifically prevent allergic reactions, steroids that modify the immune systems but may be harmful in long term use, and decongestants that reduce the symptoms.  But the best treatment is to avoid the know allergens for you.