Seasonal hayfever allergies may alter the brain

Seasonal hayfever allergies may do more than irritate the eyes and nose as they may alter the brain, a new study has shown.

Seasonal hay fever is a common problem in the UK and usually occurs in spring and summer. The condition arises when the body's immune system becomes sensitised to a number of allergens or irritants, most commonly pollens from grass and trees. Symptoms of hay fever are similar to that of the common cold and include a runny or stuffy nose, itchy eyes, sneezing, and tiredness.

Research carried out previously has suggested that allergic reactions may affect functions within the central nervous system (CNS)  and in particular, functions that are related to memory and learning. The new study, carried out by the University of Salzburg in Austria, set out to investigate this association by assessing how an allergic reaction affected the brains of mice with a grass pollen allergy compared with mice that did not.

Compared with the control mice, the brains of allergic mice demonstrated an increase in the number of neurons in the hippocampus of the brain when exposed to grass pollen. The hippocampus is the part of the brain responsible for forming new memories, and the site where neurons continue to be formed throughout life.

However, the researchers also found that an allergic reaction to grass pollen reduced microglia activity in the hippocampus of the mice. Microglia are the primary immune cells of the CNS and as such, the brain's first line of defence against pathogens.The study therefore suggests that the same allergic reaction that stimulates the body’s immune system to respond has the opposite effect on the immune cells of the brain.

This reduction was unexpected partly because previous studies have shown increased microglia activity in this brain region in response to bacterial infections.

Study co-author Barbara Klein said, "We know that the response of immune system in the body is different in case of an allergic reaction versus a bacterial infection. What this tells us is that the effect on the brain depends on type of immune reaction in the body."

The researchers say they are unable to say how these allergy-induced brain changes affect CNS function.

“Clearly, more experiments investigating the impact of different types of systemic inflammation on the CNS are needed to further the understanding about the interplay between peripheral immune activation and CNS functions," the authors conclude.

The study was published in the journal Frontiers in Cellular Neuroscience.