Embryonic stem cells, according to their genetic determination, develop into a variety of different cells that make up more than 200 different types of tissues. 

Adult stem cells don't have this ability anymore. They are responsible for the regeneration of organs and tissue. In the bone marrow, they build the red and white blood cells, the blood platelets, and the defence cells of our immune system. 

Because blood cells only have a limited lifespan, they must be replaced after a certain period of time. If this function is disrupted through an illness, the transplantation of healthy cells from a donation often helps.