What effects do seasonal allergies have on the brain?

Runny nose, watery eyes, itching: these are the most common symptoms of seasonal allergies. A recent study conducted jointly by the University and the Institute of Molecular Regenerative Medicine in Salzburg, Austria suggests they may cause more than just rhinitis and conjunctivitis; allergies may have a long-term effect on the brain, and particularly memory. For example, how can pollen affect the brain?

Currently, few findings suggest that allergic reactions can affect cognitive function in humans, though some studies have indicated that people suffering from seasonal allergic rhinitis perform less well on cognitive tests and have a greater tendency to exhibit signs of anxiety and depression. The goal of this research published in Frontiers in cellular neurosciences was to demonstrate whether or not an allergic reaction caused reactions in the hippocampus, and more specifically in the dentate gyrus, well known for its central role in cognitive functions. Barbara Klein and her colleagues hypothesized that an allergic reaction could have an effect on neurogenesis (the creation of neurons) and microglia (the brain’s immune cells).

The study was conducted on mice (ages 10-12 weeks) divided into two groups: controls (n=9) and “allergic” (n=10), in other words those sensitized to pollen and grass allergies. What were the findings?

In the "allergy" group, the researchers first witnessed a somewhat surprising increase in hippocampal neurogenesis, which could potentially have long term functional consequences on learning and memory. Secondly, to the scientists’ surprise, the allergens caused a downward regulation in microglia activity in the same region. This is precisely the opposite of what occurs during a bacterial infection. The authors believe that this could be a regulatory mechanism for protecting the hippocampus, which is central to many important processes involved in the immune response.

The Austrian team insists on the need for new research on the impact of seasonal allergies in the brain in order to better understand the interaction between immune activation and central nervous system functions. The research bears in mind a fundamental question: do the brains of allergy sufferers age more quickly?
Source: Klein B., Mrowetz H., Thalhamer J., Scheiblhofer S., Weiss R., Aigner L., “Allergy enhances neurogenesis and modulates microglial activation in the hippocampus”, in Frontiers in cellular neurosciences, 28 June 2016.

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