No one knows exactly what will happen when an experimental therapy is tried in humans.
In the twentieth century, many patients died as researchers struggled to perfect organ transplant techniques. Even today, while some transplants are routine (such as kidney), they are still not without risk. Researchers working on stem cell therapies (both embryonic and nonembryonic) worry that transplanting foreign cells could trigger an overzealous attack by the immune system (like a firefighter flooding a house for a piece of burnt toast). Another worry is that stem cells could start to grow in unexpected ways or places, establishing the wrong tissue in the wrong place.
The least regulated area of human research consists of treatments that infuse a person's own cells back into that person's body—bone marrow cells into the heart, for instance. Some scientists worry that these cells, introduced to a new environment, could do more harm than good, whereas others think even a small benefit is worth the risk and that waiting for additional studies in animals delay procedures that can benefit patients. Versions of this debate recur for all innovative therapies.
In most countries, therapies that would give patients cells from sources besides their own bodies must be tested in animals extensively before researchers receive permission to test the therapy in humans.
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What are some risks of stem cell therapies?. Nat Rep Stem Cells (2007). https://doi.org/10.1038/stemcells.2007.26