Hay fever — also called allergic rhinitis — is triggered by either or both indoor and outdoor allergens, including pollen, dust mites and pet dander. Symptoms can include everything from runny nose to stuffy nose, itchy eyes and congestion to sneezing and painful sinus pressure.
Now, it turns out, hay fever and other seasonal allergies may also change the brain.
In a study published in the open-access journal Frontiers in Cellular Neuroscience, scientists found that brains of mice exposed to allergens actually produced more neurons than controls.
The research team examined the hippocampus — the part of the brain responsible for forming new memories and the site where neurons continue be formed throughout life. During an allergic reaction, there was an increase in the numbers of new neurons in the hippocampus, which made the scientists question whether allergies can affect memory.
The formation and functioning of neurons is linked to the brain's immune cells, the microglia. To the scientists' surprise, they found that the same allergic reaction that kicks the body's immune system into high gear has the opposite effect on the brain's resident immune cells. The microglia in the brain were deactivated in brains of these animals.
"It was highly unexpected to see the deactivation of microglia in the hippocampus," explained Barbara Klein, one of the authors of the study. "Partly because other studies have shown the reverse effect on microglia following bacterial infection. 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 research team feels that, based on its initial findings, it would be worth investigating "what allergy does to microglia in the aged central nervous systemp, which might already be somewhat primed for a pro-inflammatory activation."
Allergic reaction also causes an increase in neurogenesis — the growth and development of nervous tissue, which is known to decline with age. The team said it would also be worth investigating whether alternative sensitization and challenge routes (e.g., in a model for food allergy) have a similar impact on microglia and neurogenesis in the hippocampus.
"So far, we can only hypothesize that the observed increase in hippocampal neurogenesis may also have functional consequences on learning and memory. For this, further studies including electrophysiological analysis and behavioral tests are needed," according to the research team.
The study has highlighted the need to further examine inflammation and its diverse effects — which are dependent on the type of immune response — on our bodies, particularly the central nervous system. At the very least, it may help allergy sufferers feel like all their misery is not entirely in vain.