Deanne Stein, NewsOn6.com

OKLAHOMA CITY -- Grab a tissue, it's that time of year again. Fall allergies are here and causing many Oklahomans to suffer through another round of sniffles.

The cooler temperatures typically cause runny noses, watery eyes and sniffling. But not all of it is due to colds and viruses.

"We may think of autumn as prime time for colds and flu, but it's also allergy season for many Oklahomans," said Oklahoma Medical Research Foundation President Stephen Prescott, M.D.

Ragweed is the main culprit and especially plentiful in this region of the country. Its pollen can trigger allergic reactions until the first freeze. This month, Asthma and Allergy Foundation of America ranked Oklahoma City and Tulsa 5th and 10th, respectively, on its list of the worst cities for fall allergies.

But that ragweed and pollen may not be the only reason you have a stuffy nose, Prescott said your immune system plays an important part is how much you could suffer.

"Allergies occur when the immune system overreacts to substances that most people's bodies perceive as harmless," he said.

While the immune system is supposed to fight off sicknesses, it can sometimes get confused. In fact, for people with allergies, the confusion begins when the body senses a substance like pollen or dust and thinks it's found a dangerous intruder.

"It's the difference between coming home and finding a stranger with a gun in your house or finding a stray cat inside," said OMRF immunologist Hal Scofield, M.D. "Neither are welcome guests, but we react to the situation differently."

When those allergens enter the body, the immune system attacks with antibodies. When the antibodies bind with the allergens, it releases histamine into the system. Histamines then interact with the nasal mucous membrane, causing watering eyes, congestion and runny nose.

"Like most chemicals in the body, histamines do several things—one of which is keep us awake," Scofield said. "That's why you have to be careful with antihistamines, which cause drowsiness."

For those with fall allergies, Scofield recommends limiting exposure to the outdoors. And before prolonged outdoor events, he suggests using nasal washes and pre-medicating with anti-allergy medications.

"With this year's drought, allergies are likely to be worse than usual, as plants will spread more pollen to improve their chances of reproduction," he said. "So fall allergy sufferers will want to be extra vigilant."