AH, MY SWEET summer child. What do you know of inflammation? Inflammation is for the winter, when genes uncoil in your blood and messengers send codes containing the blueprints for proteins to protect you from the harsh diseases of the cold. Inflammation is for those long nights, when the sun hides its face, or rain clouds block the sky, and trillions of little T-cells are born to fight the diseases of cold and flu season.

At least, that’s the news from a new study showing that DNA reacts to the seasons, changing your body’s chemistry depending on the time of year.

The findings, published today in Nature Communications1, show that as many as one-fifth of all genes in blood cells undergo seasonal changes in expression. Genes often are seen as immutable, but a lot of our body’s workings depend upon which genes are translated when. In the winter, the study found, your blood contains a denser blend of immune responders, while summer veins swim with fat-burning, body-building, water-retaining hormones. These seasonal changes could provide insight into inflammatory diseases like hypertension, and autoimmune diseases like type 1 diabetes.

The question is slap-your-forehead simple: How do genes respond to seasonal changes? Surprisingly, nobody had yet looked. “We knew that there are some genes that change their expression throughout the day, and then it hit us—like BLAM!—what is the effect on genes of the length of day throughout the year?” says Chris Wallace, an immunogeneticist at Cambridge University.

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