Pollen, dust mites, cats, bee stings. We think of these as common allergy triggers. But exposure to ANY substance may cause an allergic response in someone, OR PREVENT it. SOMETIMES it's the LACK of exposure to an allergen that increases the risk of a more severe attack once exposure does happen.An allergic reaction occurs when the immune system over-reacts to a normally harmless substance, and identifies it as HARMFUL. This causes it to create antibodies to the substance. The end result? The release of inflammatory histamines that can trigger the sneezing, itchy eyes and runny nose that are common signs of an allergy attack. Often, you have to be exposed to the allergen at least ONCE before the allergy develops. That's because the first exposure is when the immune system reacts to the allergen and creates antibodies to fight it. For some people though, it takes more than one exposure. And in fact it can sometimes take years. Some adults, who never were allergic to pollen, only develop symptoms after years of repeated exposure. Interestingly, repeated EXPOSURE can also have the REVERSE EFFECT. It may help the immune system develop immunity to an allergen. Sometimes a person won't have an allergic reaction to his or her own dog or cat, but will be allergic to someone else's. LACK of exposure may also make the body more susceptible to allergies. According to researchers, individuals living in too antigen-free an environment may not be exposed to enough pathogens to keep the immune system busy. As a result their system over-reacts to exposure to a normally harmless substance such as pollen. This hypothesis has been used to explain the increase in allergies and autoimmune diseases, especially in more developed countries.How your body reacts to exposure is unpredictable and you cannot know what will help prevent or trigger an allergy until you look back on a situation and figure out exactly what happened-if you can! To learn about allergy treatments and prevention, check out other videos in this series.