Does exercise change our immune cells, the cells that fight germs? Scientists in China looked at the blood of 86 young adults. Forty of them exercised often, and 46 did not exercise.

The scientists studied the blood one cell at a time. In people who exercised, many immune cells seemed ready to fight quickly. Their immune cells also showed weaker inflammation signals. But this study does not show that exercise caused these changes.

Forty young adults in China who exercised regularly gave blood samples, and so did 46 people who rarely moved. A research team compared the two groups to see what exercise does inside our immune cells. This kind of study has a weak point.

Each person was tested only once, so it cannot prove that exercise caused any difference it finds. Even so, the team found clear differences. Using new technology, they examined immune cells one by one.

In the active group, the parts of DNA used for fighting infections were more open and ready to be read. It was as if the cells had prepared in advance. Signals between immune cells were also stronger. Networks linked to inflammation, by contrast, were weaker.

The 86 volunteers in this study averaged about 28 years old. Forty met the WHO's recommended activity levels, and 46 were sedentary. Researchers in China, writing in Science Advances, drew on a large immune database to compare the two groups. They combined analysis of blood metabolites with single-cell measurements of gene activity and of chromatin accessibility, which shows how open each stretch of DNA is.

The active group stood out in several ways. Their antigen-presenting cells were more active in genes used to display signs of invaders, and their killer T cells and mature natural killer cells appeared to be primed in advance, a kind of standby state. Immune cells communicated more with one another, inflammatory networks were dampened, and the blood carried more molecules related to fat burning and antioxidants.

What the design allows us to conclude is another matter. Everyone was sampled at a single point in time, exercise levels were self-reported, and the groups differed in sex balance, and other differences between them may remain. Only a trial that gets people to start exercising could show that the workouts themselves create the standby state. And nobody checked whether the active volunteers actually caught fewer infections.