Our immune systems vary with the seasons, according to a study led by the University of Cambridge that could help explain why certain conditions such as heart disease and rheumatoid arthritis are aggravated in winter whilst people tend to be healthier in the summer. The study, published in the journal Nature Communications, shows that the activity of almost a quarter of our genes (5,136 out of 22,822 genes tested) differs according to the time of year, with some more active in winter and others more active in summer. This seasonality also affects our immune cells and the composition of our blood and adipose tissue (fat).

Scientists have known for some time that various diseases, including cardiovascular disease, autoimmune diseases such as type 1 diabetes and multiple sclerosis, and psychiatric disorders, display seasonal variation, as does vitamin D metabolism. However, this is the first time that researchers have shown that this may be down to seasonal changes in how our immune systems function.

"This is a really surprising -- and serendipitous -- discovery as it relates to how we identify and characterise the effects of the susceptibility genes for type 1 diabetes," says Professor John Todd, Director of the JDRF/Wellcome Trust Diabetes and Inflammation Laboratory. "In some ways, it's obvious -- it helps explain why so many diseases, from heart disease to mental illness, are much worse in the winter months -- but no one had appreciated the extent to which this actually occurred. The implications for how we treat disease like type 1 diabetes, and even how we plan our research studies, could be profound."