The tree · Replacing lost cells and parts
An old animal's blood-making stem cells are made young again, and its immune system with them
The stem cells in bone marrow that make every blood and immune cell change with age: more of them come to favour the inflammatory cells of the innate immune system and fewer the T and B cells that learn new threats, which is one reason old people respond poorly to vaccines and infections. In 2024 a Stanford group used antibodies to kill off the age-biased stem cells in old mice, and the mice's marrow shifted back toward a young mix, with better responses to a virus and less inflammation. The same biased cells carry the same surface markers in old people. It is the first rejuvenation of the immune system's source, rather than a top-up of its products.
What would count
An old living animal given a treatment that removes or resets its age-biased blood stem cells afterwards has a measurably younger mix of blood and immune cells and a better immune response, compared with untreated old controls, in a peer-reviewed report. The first-human rung is the same in a person. For the clinic rung: a regulator approves such a treatment for an age-related condition.
Who settles it: Peer-reviewed reports; the trial registry and report when a first person is treated; the regulator's approval record for the clinic rung.
Where it stands
Shown in animals. Reached 27 March 2024, on the record of doi.org. Next rung more than 20 years away, if ever.
When it reaches the clinic: the project's estimate
This is the one thing on this page that is a judgment rather than a record. The project's estimate of the year it reaches the clinic is 2048, no earlier than 2036 and no later than 2100; the model draws from that range every run, with the likely year the most probable draw. Set 16 October 2026, a first pass awaiting an outside review.
| Rung | No earlier than | Most likely | No later than | |
|---|---|---|---|---|
| First human | 2029 | 2035 | 2055 | |
| In clinical use | 2036 | 2048 | 2100 | feeds the year at the top |
Why. The 2024 result named human surface markers, so an antibody for people is makeable; the likely year for a first person is the mid 2030s through a condition such as a blood cancer precursor where removing biased stem cells is already wanted, and the clinic rung around 2048 after a trial in that condition. The earliest assumes an existing antibody is repurposed within a few years; the latest allows for the marrow proving too risky a target in older people.
Changes to this record
None since it was written on 16 October 2026. Every change to the stage or the estimate lands here with its date, the old value, the new one and the reason, and a change to the estimate never happens because a number looked wrong: it needs a dated reason and a source.
The last readings
- 17 October 2026 Quiet
Nothing in the window. The searches returned the 2024-03-27 Nature paper the record rests on, no registered trial or first person, and one older related mouse paper: a Lund University group (David Bryder) reported in Nature Communications that clearing old mice's own blood stem cells with an antibody-toxin and transplanting young ones restored naive B and T cell output without chemotherapy; the search results dated the university's release to 2025-09-13 and a repost to 2026-07-23, so it is before the window either way, it is a transplant rather than a reset of the animal's own cells, and it bears on the animal rung the leaf has already reached, not on the first-human rung.source 1, source 2
What the story so far says about it
No promise, self-experiment or logged paper names this one yet.
Notes
Immune rejuvenation has two targets, the thymus (where T cells are trained, and which shrinks from puberty on) and the marrow (where the cells come from); this leaf is the marrow, and the thymus result is on the cell-settings branch because it was measured with a clock. The 2024 treatment killed cells with antibodies, which in a person would be done in a hospital like a mild chemotherapy, so the clinic rung is a long way off.