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Some diseases have a higher incidence in certain seasons. For example, the incidence of cardiovascular disease deaths, rheumatoid arthritis, type 1 diabetes, and neuropsychiatric conditions is greater in winter. These diseases also are characterized by inflammation.
A European team examined thousands of blood samples from ethnically diverse populations in the Northern and Southern hemispheres and the equatorial region to see if seasonal differences could be identified in the immune system. They measured both the expression of immune system–related genes (as reflected in mRNA levels) and proteins linked to inflammation. About 23% of the human genome exhibits significant seasonal variation: Some genes are upregulated in summer, and others prod…