Inside your bones sits a soft tissue called bone marrow. It works like a factory that churns out billions of blood cells every day: red cells carry oxygen, white cells fight disease, and platelets heal wounds. When you are young, the factory runs like clockwork. But with age, broken machines start piling up on the floor: senescent cells. They produce nothing useful, yet they release harmful substances that get in the way of the healthy cells. Because of them, the factory starts shipping out too many inflammation cells and too few immune cells. That is why older people get sick more often and take longer to recover.

Scientists decided to clear out the broken machines. They worked with mice and used two approaches. The first was genetic: the mice were engineered so that senescent cells could be killed with a single drug. After that cleanup, the blood factory ran better: stem cells, the master craftsmen on the factory floor that give rise to all the other cells, divided more often and produced the right mix. The second approach was a new drug, 753b. It acts like a precision tool that destroys only senescent cells, leaving healthy ones untouched. Old mice were given eight injections of this drug, and the result was similar: the balance of blood cells was restored, and the stem cells looked younger.

Earlier drugs of this kind were dangerous because they lowered platelet levels, and the blood stopped clotting. The new drug 753b does not do that: in old mice after treatment, platelets reached the same levels as in young mice, but no lower. That means the drug can be used long term. If human trials go well, doctors will be able to treat age-related anemia, weak immunity and, possibly, prevent blood cancers. The blood factory will run like it did in youth.

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