The immune system plays an important role in the heart's response to injury. Now, researchers at Washington University School of Medicine in St. Louis have shown that two major pools of immune cells are at work in the heart. Both belong to a class of cells known as macrophages. One appears to promote healing, while the other likely drives inflammation, which is detrimental to long-term heart function. Studying mice, new research suggests that embryonic macrophages in the heart promote healing after injury.