There is no commodity more precious than blood, since having enough is the difference between life and death, Too often, hospitals have not had enough. Since the seventeenth century, doctors have experimented with blood substitutes, giving transfusions of everything from animal blood to oil and even milk to hapless patients, but a safe and effective substitute has remained elusive until just recently.
   In May of 1999, doctors at an American hospital attempted a radical new procedure on a patient whose immune system had mysteriously begun to attack her red blood cells. With no known way to counter the patient's loss of red blood cells, doctors had little hope for the dangerously anemic young woman. In one last desperate attempt, Dr, George Giacoppe, head of the intensive-care unit, contacted one of the companies attempting to develop an effective blood substitute.
   With the patient near death, Giacoppe obtained permission from authorities and began giving his patient transfusions of Hemopure, an experimental blood substitute derived from highly purified cow's blood. Chances for success were low-the blood substitute was still in clinical trial, and no substitute had ever passed this stage. Amazingly, it worked. Hemopure carried badly needed oxygen to the patient's anemic cells without provoking an attack from her immune system. A few hours later, she was sitting up und chatting with doctors.
   If health regulators give the go-ahead for blood-substitute usage, it will be none too soon. Due to the fear of contamination from such diseases as AIDS and mad cow disease, restrictions on who can donate blood have been expanding steadily. As a result, blood is in short supply. Moreover, the substitutes also promise several benefits over normal blood transfusions. They do not spoil as quickly as human blood and do not require refrigeration. Doctors would no longer have to worry about the deadly mistake of mismatching the blood types of donors und patients. And, importantly, substitutes are less likely to be infected by human viruses.
  It was once believed that the need for new blood sources would spur faster approval of substitutes by the FDA, America's main health regulatory agency. But the head of that agency, Abdu Alayash, says that speedy approval is not necessarily desirable. The substitutes now being considered still do not perfectly replicate the role of blood, While they do carry oxygen even more efficiently than red blood cells, they do not offer the protective role of white blood cells, which fight disease, The oxygen-carrying particles in the substitutes arc much smaller than red blood cells, which enables them to slip through clogged arteries-a real benefit. But researchers wonder if these same properties may also cause negative side effects.
   Researchers are also concerned about the apparent tendency of　substitutes to increase blood pressure in recipients. “These products　could do harm, and we have nothing to compare them with,” said Alayash. He believes that cases like those of Dr. Giacoppe's patient　are no substitute for long-term, in-depth clinical trials that test hundreds of patients.