London: Seasonal allergies like hay fever reduce the presence of brain immune cells as well as increase the growth of nervous tissue, which may lead to changes in the brain, says a study. Hay fever, also known as allergic rhinitis, is caused by an allergic response to outdoor or indoor allergens such as pollen, dust mites and leads to cold-like runny nose, itchy eyes, congestion, sneezing and sinus pressure. According to a report by the World Health Oragnisation (WHO), 10 to 30 per cent of the population worldwide suffers from hay fever. The findings showed that brains of mice when exposed to a model of grass pollen allergy actually produced more neurons than when they were under normal conditions. During an allergic reaction, an increase in the numbers of new neurons in the hippocampus -- the part of the brain responsible for forming new memories, and the site where neurons continue to be formed throughout life -- was found. This raised the question that what could be the consequences of allergies on memory, the researchers said. In addition, the allergy also reduced the presence of microglia -- brain immune cells that perform immune system functions in the central nervous system - activity. The microglia in the brain of mice were found deactivated. "It was highly unexpected to see the deactivation of microglia in the hippocampus. Partly because other studies have shown the reverse effect on microglia following bacterial infection," said Barbara Klein, University of Salzburg in Austria. "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," Klein added. The allergic reaction also caused an increase in neurogenesis -- the growth and development of nervous tissue, which is known to decline with age, said the paper published in the journal Frontiers in Cellular Neuroscience.