Allergies occur when the body's immune system reacts to a substance, known as an allergen, that is usually harmless. Common allergens include pollen, dust mites, pet dander, and certain foods like peanuts or shellfish. When someone with allergies comes into contact with these substances, their body may produce symptoms such as sneezing, itching, or even more severe reactions. People can experience different types of allergies, including seasonal allergies that happen during specific times of the year, and food allergies that can cause serious health issues. Managing allergies often involves avoiding allergens and using medications like antihistamines to relieve symptoms.