Early infections may “prime” a child’s immune system and help prevent allergies and conditions like asthma, research by US and Japanese experts found.
The scientists analysed adult mice who had been infected with the influenza A virus as a baby.
Adult animals catching the flu virus remained susceptible to asthma, while those infected when they were young were more protected.
The scientists traced the effect to a sub-group of immune system cells called natural killer T-cells (NKT cells).
Some NKT cells appear to keep the immune system under control and prevent the extreme reactions that lead to allergies such as asthma. In the baby mice with flu, but not the adults, numbers of these cells were found to increase.
The research also showed that NKT cell protection against asthma could be induced by exposing baby suckling mice to a molecule from the stomach bug Helicobacter pylori.
The findings, published in the Journal of Clinical Investigation, provide strong evidence in support of the “hygiene hypothesis”, the US and Japanese scientists believe.
This suggests that sharply rising rates of asthma in developed countries over the past few decades may be linked to reduced childhood exposure to bacteria or viruses.
The scientists, led by Dr Dale Umetsu, from Harvard Medical School in Boston, US, wrote: “Our results suggest that infection with certain micro-organisms can prevent the subsequent development of asthma and allergy by expanding the relative proportion of a specific subset of NKT cells, thus providing an immunological mechanism for the hygiene hypothesis.”