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Arthur T Knackerbracket has found the following story:

In your body, blood stem cells produce approximately 10 billion new white blood cells, which are also known as immune cells, each and every day. Even more remarkably, if some of these blood stem cells fail to do their part, then other blood stem cells pick up their slack and overproduce whichever specific type of immune cell is lacking, according to a new USC Stem Cell study published in the journal EMBO Reports.

USC PhD student Lisa Nguyen and colleagues in the laboratory of Rong Lu observed this phenomenon by tracking the individual blood stem cells that reside in the bone marrow of mice. To create the tracking labels, the scientists attached a unique piece of genetic code to each blood stem cell. During blood production, each blood stem cell passes its unique genetic label onto its progeny -- which include two types of immune cells, known as B cells and T cells.

[...] The scientists found that the normal blood stem cells compensated for the B and T cell deficiencies. When co-transplanted with B-deficient stem cells, the normal stem cells overproduced B cells to keep the immune system in balance. And when co-transplanted with B- and T-deficient stem cells, the normal stem cells compensated by overproducing both B and T cells to maintain a balanced immune system.

Furthermore, the scientists found that a few specific blood stem cells were doing most of the work. These key blood stem cells proliferated dramatically to compensate for the immune cell deficiencies in the recipient mice, and these cells continued to proliferate when they were transplanted into different recipient mice. Furthermore, these highly productive blood stem cells showed changes in gene activity that enhanced their ability to oversupply deficient types of immune cells.

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