WHAT ARE STEM CELLS? Stem cells are distinguished from other cells in the body in two ways. First, they are unspecialized cells -- i.e., they do not begin with any specific function within the body -- and can renew themselves through cell division over long periods of time. Second, it is possible to induce them to turn into cells with specific functions. Scientists want to study stem cells in the laboratory in order to learn what makes them different from other specialized types of cells. Specifically, they would like to know how they remain unspecialized and self-renewing for so many years, and what signals within the body cause them to turn into specialized cells. By doing so, it may one day be possible to use stem cells in cell-based therapies, as well as for screening new drugs and toxins, and for gaining a better understanding of birth defects.
ABOUT TRANSPLANT REJECTION: The human body's immune system is designed to protect the body from potentially harmful agents, such as viruses, bacteria, toxins, and cancer cells. These are known as "antigens." The body can distinguish itself from unfamiliar substances because blood and tissue have specific proteins on the surface. These proteins can act as antigens and trigger an immune response, namely, the production of antibodies to fight foreign invaders. That's what happens with organ rejection: the body doesn't recognize the new organ as "itself" and begins attacking the new organ as though it were an invading micro-organism. The new organ can be damaged or destroyed. No two people have exactly the same tissue antigens, except for identical twins. So before a transplant is done, the tissue of donor and recipient is typed to find the closest possible match. The greater the difference, the more rapid and severe the rejection. Certain kinds of drugs, called immunosuppressants, are also used to reduce the immune system's reaction to the new organ, and can often stop rejection, although the patient becomes more susceptible to infection. These include steroids and drugs such as cyclosporine and azathiaprine.