Newborns with heart complications can rely on their newly developed immune systems to regenerate cardiac tissues, but adults aren’t so lucky. After a heart attack, most adults struggle to regenerate healthy heart tissue, leading to scar-tissue buildup and, often, heart failure. A new study in experimental animals reveals a critical difference in how macrophages — a part of the immune system — help repair the heart in newborns versus adults after a heart attack. The study highlights a fundamental difference in how the immune system drives healing based on age.