You may have been able to avoid allergies for decades, when suddenly at age 20 or 30 or older, you develop them. It hardly seems fair, but according to research, adult onset allergies are increasingly common. But why? Research suggests that many adults actually had allergies as a child, but may never have known, or have forgotten about them. The allergies subsided as they grew up. But now, as people get older and their immune system weakens, the allergies flare up again. For other adults, repeated exposure to allergens finally sensitizes their immune system. They develop an allergy to cats, for example, after having been a long-time cat lover. Allergies can also develop if a weakened immune system is exposed to allergens. This may occur after a viral infection or during pregnancy. In fact, it's pretty common for women to develop allergies after pregnancy. Then there's the hygiene hypothesis. This theory claims that our immune system - these remarkable engines designed to fend off assaults from germs - are underused these days. Due to an overuse of sanitizing agents, antibiotics, and our highly germ-free environment, our immune systems don't come in contact with the disease-causing antigens that our ancestors did. That causes our underused immune systems to turn on own bodies, creating allergies and autoimmune diseases. The immune system is acting like a bored adolescent, getting in trouble just because there doesn't seem to be anything else to do. Luckily, adult onset allergies - including allergic asthma - can be treated with antihistamines, antileukotrienes, and nasal corticosteroid sprays. And immunotherapy with shots or sublingual drops can help reset the immune system so the allergic reaction is reduced or even prevented. Only a board certified allergist can determine what form of immunotherapy is best for you, depending on the type of allergies you might have as well as their severity. To learn about other common allergies, check out more videos in this series.