An allergy is a specific reaction of the body's immune system to a normally harmless substance, one that does not bother most people. People who have allergies often are sensitive to more than one substance. Types of allergens that cause allergic reactions include:
 

• Pollens
• House dust mites
• Mold spores
• Food
• Latex rubber
• Insect venom
• Medicines

Courtesy: National Institute of Allergy and Infectious Diseases