When your freshman comes home from college this summer--and you happen to have a dog or a cat--don't be surprised if she starts sniffling and sneezing for the first time ever. Some call it the college phenomenon, others have dubbed it the Thanksgiving Effect...you send your child to college seemingly allergy free and she comes home allergic to the pet she grew up with.

What's going on?

The phenomenon was reported at the Congress of the European Academy of Allergy and Clinical Immunology in Warsaw, Poland earlier this month. Though somewhat a mystery, it's thought that when you're exposed to dogs and cats--and other allergens--a couple of things happen. Your inflammatory system may become less responsive, and you also churn out IgG, a type of antibody that helps fight infections and plays a role in allergies, but does not trigger those classic allergic symptoms. IgG antibodies are thought to protect against allergies by eliminating the allergen from your body and keeping your inflammatory cells from becoming activated. Some people also develop IgE, the antibodies that trigger runny nose, itchy eyes and other allergic symptoms. But because of the protection offered by the IgG antibodies, IgE does not function efficiently enough to mount an allergic response.