As we age, our immune system quietly loses its edge, and scientists have uncovered a surprising reason why. A protein called platelet factor 4 naturally declines over time, allowing blood stem cells to multiply too freely and drift toward unhealthy, mutation-prone behavior linked to cancer, inflamma tion, and heart disease. Researchers found that restoring this protein in older mice — and even in human stem cells in the lab — made aging blood and immune cells behave strikingly younger again. ...read more read less