Allergies are the immune system's overreaction to certain substances that it deems dangerous, even though they're perfectly safe. When an allergen triggers your immune system, your body releases immunoglobulin E, which then tells basophils and mast cells to send out chemicals to fight the "invader." Many people are sensitive to allergens released by warm-blooded animals, including the proteins in their saliva, dander and fur. Reactions can include congestion, itchy eyes, runny nose and sneezing. The best way to avoid allergic reactions is to avoid contact with the allergens -- in this case, pets.
If you can't avoid contact with animals or their dander, doctors will sometimes suggest immunotherapy, which is the administration of allergy shots. Allergy shots are typically recommended when your pet allergies are very severe and the benefits of the shots outweigh the cost and time you have to invest in the immunization process. Other considerations include whether medications that treat the symptoms are effective for you and whether the allergy might trigger other conditions, such as asthma.
Allergy shots normally continue to work for as long as you take them. Immunotherapy works via the injection under your skin of small amounts of dander extract mixed with saline. By repeating the process once a week and increasing the dosage for about half a year, your body becomes used to the allergen and stops reacting when you encounter pets. At that point, you can slow down the shots and get one every two to four weeks for another six months. If you're still having allergic reactions to your pets after a year, the doctor will most likely recommend that you give up (and possibly give up your pet). If you're not having reactions, you can keep getting the shots for up to about five years.