Sneezing, stuffy nose, itchy eyes: it’s that time of year when hay fever reaches its peak, causing millions of Americans to reach for the antihistamines. But according to a new study, this seasonal allergy may do more than irritate the eyes and nose; it could alter the brain.

Researchers suggest seasonal allergies might increase neurogenesis and reduce immune activity in the brain.

In mouse models of grass pollen allergy, researchers found that exposure to an allergen led to both the development of new neurons – or nerve cells – and reduced activity of immune cells in the hippocampus of the brain.

The hippocampus is the brain region important for memory and learning.

Study co-author Barbara Klein, of Paracelsus Medical University in Salzburg, Austria, and colleagues recently published their findings in the journal Frontiers in Cellular Neuroscience.

According to the American College of Allergy, Asthma, and Immunology, around 40-60 million people in the United States have hay fever, also known as allergic rhinitis.

Seasonal hay fever usually occurs in spring, summer, and early fall. The condition arises when the body’s immune system becomes sensitized to a number of allergens or irritants, most commonly pollens from grass, trees, and weeds.

Symptoms of hay fever include runny or stuffy nose, itchy eyes, mouth, or skin, sneezing, and fatigue.

Klein and colleagues note that previous research has suggested allergic reactions may affect functions within the central nervous system (CNS) – particularly functions related to memory and learning.

The team further investigated this association by assessing how an allergic reaction affected the brains of mice with a grass pollen allergy. Their brains were then compared with mice that did not have a grass pollen allergy – the controls.

Compared with the control mice, the brains of allergic mice demonstrated greater neurogenesis in the hippocampus of the brain when exposed to grass pollen.

What is more, the researchers found that an allergic reaction to grass pollen reduced microglia activity in the hippocampus of the rodents. Microglia are the primary immune cells of the CNS; they are the brain’s first line of defense against pathogens.

Klein says they were surprised to find reduced microglia activity in the hippocampus due to an allergic reaction, partly because previous studies have shown increased microglia activity in this brain region in response to bacterial infections.

“We know that the response of immune system in the body is different in case of an allergic reaction vs. a bacterial infection. What this tells us is that the effect on the brain depends on type of immune reaction in the body.”