The immune system is central to human health. When it fails, people can become more susceptible to infection and cancer. When misdirected, it can attack the body’s own tissues. And as it ages, it can contribute to chronic disease.

People of the same age can have strikingly distinct immune systems, shaped by genetics and by environmental and lifestyle factors, including air pollution, smoking, diet and past infections 1 , 2 . Growing evidence suggests that variation in the immune system’s ‘biological age’ — the rate at which it has aged, which is distinct from chronological age — matters for health . This variation has been linked to neurodegeneration, cancer and mortality risk 3 .

Immune cells have a surprising role in exercise endurance

Yet, clinicians have no routine way of measuring how healthy someone’s immune system is. That’s because the immune system is a dynamic network that can change over time.…