Types of Allergies ~ An allergy is a type of reaction of the immune system. Substances that often cause allergic reactions are pollen, dust mites, mold spores, pet dander, food, insect stings and medicines. Allergies can cause a running nose, sneezing, coughing, itching, skin rashes or asthma. Symptoms vary from person to person and with the type of allergen. Rarely, allergies can set in a severe reaction called anaphylaxis, which is life-threatening.