Immune System SeriesThe Stem Cell

Scientists have long sought the hematopoietic stem cell, the precursor cell that continuously replenishes the body's entire panoply of blood cells, both red and white. Stem cells represent a small portion of all bone marrow cells (perhaps one in 2,000), and they are even rarer in the bloodstream. In the mouse, implanting just a few purified stem cells can completely restore an immune system that has been experimentally destroyed.

Although the human stem cell has yet to be isolated and purified, scientists have discovered that progenitor cells capable of giving rise to an array of blood cells (if not of actually reproducing themselves) carry the cell surface marker CD34. These cells can be sorted out from marrow and blood with monoclonal antibodies that recognize CD34. In experimental programs, CD34 cells are being tested as long-lived vehicles for gene therapy and as an alternative to bone marrow transplants.