Sometimes, allergies show up almost immediately. Other times, they form over long periods of close exposure to the allergens. It's this later issue that can be a big problem for scientists and their ability to work with certain laboratory animals. Fifteen to 20 percent of scientists who work with mice and rats may eventually become allergic to those animals, writes Hilary Rosner in The New York Times. So what happens when your immune system affects your ability to do your job?