The I-cells, along with all the other red and white blood cells, develop from cells in the marrow called stem cells. The stem cells, in turn, evolve into progenitor cells. It is the progenitor cells that differentiate into the various red and white blood cells.

A researcher at the University of Illinois at Urbana-Champaign, Dr. Keith Kelley, has found, in some of his most recent work, that the progenitor cells in both older humans and older rodents, were in serious decline. When he gave growth hormone to the old rats, the progenitor cells showed a marked resurgence and regained their numbers.

Since stem cells have the capacity to outlast the human life span, Dr. Kelley believes that the problem of diminished progenitor cells does not originate with the stem cells, but elsewhere in the production line. There is some mechanism in the production line that becomes blocked or defunct with age. But whatever that blocked mechanism is, growth hormone is able to help set it right. In Dr. Kelley's words:
"Somehow the hormones act to overcome a block... We don't know exactly what happens. Magic occurs."

No doubt the details of this striking discovery are currently being sought out through further research. Meanwhile, this "magic" leaves us with an irresistible chain of logic: Humans and rodents suffer diminished immune capacity as they age. And as they age humans and rodents suffer diminished numbers of progenitor cells (which are the cells that give rise to our army of disease fighters). Growth hormone brings back the number of progenitor cells in aging rodents. Therefore, since growth hormone does strengthen the human immune system, it seems likely that one way in which it does this is by bringing back the numbers of progenitor cells in aging humans.