Allergies may seem straightforward: You're exposed to allergens like dust, mold or animal dander, you get watery eyes, you sneeze and you feel miserable. But you may be surprised to learn why some people suffer more than others. Here, 10 shocking facts about what may really be making you feel bad.

1. If you're the oldest in your family, you may be more prone to allergies.
Researchers in the UK recruited more than 1,200 children, testing them for signs of allergies at birth, age 4 and age 10. They found markers of allergies in firstborns, but not in later-born children, likely because firstborns experience different conditions in the womb, researchers say.

2. It may not be your pet that's causing your allergies.
"It could be something else entirely," says Jeff May, an indoor air quality expert and coauthor of Jeff May's Healthy Home Tips. "Dust mites from your animal's bed (or yours), or mold underneath antiques in small spaces" are possible culprits, he says. Even homes without pets can have an pet allergen problem. In fact, a study from the National Institute of Environmental Health Sciences reports that more than 99 percent of homes in the U.S. have detectable cat or dog allergens even though less than half of those homes have a dog or cat living there. Allergens can hitch a ride on other people's clothes and they've been discovered in schools, hospitals, shopping malls, cinemas-and even in the offices of allergists.

3. Timing your meds right can make all the difference.
"Your body follows what's called a circadian rhythm-it acts differently at different times of the day," says Richard Martin, MD, head of the pulmonary division at National Jewish Medical and Research Center in Denver, CO. This natural rhythm influences when allergies worsen, and therefore determines when it's best to take medication to achieve its maximum effect. For example, if you have 24-hour meds, you should take them at night. "Allergy symptoms peak in the morning hours, so this ensures they'll be moving through your bloodstream when needed most," he says.

4. Obese children and adolescents have a higher risk of allergies.
Especially food allergies, a study in the Journal of Allergy and Clinical Immunology notes. Researchers analyzed data from over 4,000 subjects ages 2 to 19, and discovered that obese participants were about 26 percent more likely to have allergies than children of normal weight. The rate of food allergies was especially troublesome-59 percent higher for children who were obese.

5. Your sneezes around your daughter's new kitten may be more than just a cold.
"You can grow up with pets and still develop allergies to them as an adult," says Morris Nejat, MD, of the New York Allergy and Sinus Centers. "Having a pet gives you an immunity that can be lost when you leave that environment-to move away and start your own life, for example. If you then bring an animal into your own home, you may have lost your immunity and developed an allergy to your new pet."

6. Plants aren't as healthy as you think.
Unfortunately, "plants do not purify the air," May says. "Put a thin layer of gravel on top of each plant's soil-this helps keep debris in the plant, protecting you from mold allergens," recommends James M. Seltzer, MD, chairman of the Indoor Environment Committee of the American College of Allergy, Asthma & Immunology. "Even silk plants can cause mold," Dr. Seltzer says.

7.Trouble breathing? Don't look to your lungs, look to your gut.
Upsetting the normal balance of microflora in your stomach and intestines can change your entire immune system, researchers at the University of Michigan Medical School report, intensifying your body's response to common allergens like pollen or animal dander. According to scientists, our modern diet and increased use of antibiotics may be at fault. Safeguard your health with a diet low in sugar and high in raw fruits and vegetables, especially following treatment with antibiotics.

8. Allergies can trigger bad breath.
"For some people, a dry mouth, caused by taking antihistamines for allergies or by the postnasal drip that streams down the back of your throat during allergy attacks, can result in bad breath," says Cyril Meyerowitz, DDS, director of the Eastman Institute for Oral Health, part of the University of Rochester Medical Center in Rochester, New York. "Sometimes tiny pieces of food lodge on the tongue or in the crevices of your tonsils and cause a nasty odor."

9. You can't become immune to your allergy meds.
"You may feel that your allergy medication has become ineffective, but there's no scientific basis for medication tolerance," says Linda B. Ford, MD, medical director of the Sarpy/Cass Department of Health and Wellness at the Asthma and Allergy Center in Papillion, Nebraska. "What's more likely is that the disease has become more severe, in which case you should consider other treatment options, like allergen immunotherapy, intranasal steroids, isotonic saline nasal washes and/or other antihistamines."

10. Your allergies may have an upside.
A study from the Harvard School of Public Health in Boston reports that people with a history of allergies have nearly a third lower risk of developing a type of brain cancer called glioma than those without the condition. Researchers suspect this may be due to the genetic variations that cause the allergies.