Blood substitutes would eliminate the need to match blood types before transfusing trauma victims and eliminate the risk that HIV, hepatitis viruses and other pathogens could be transmitted by transfusions. But clinical trials of blood substitutes have been marked by poor performance and often have been cut short because of nasty side effects. Though the latest generation of substitutes to use fluorocarbons looks promising, Klein said previous versions have caused flu-like symptoms, mild fevers and a drop in platelet counts. Patients receiving the substitutes suffered higher death rates. Genetically engineered hemoglobins also have been tested as substitutes, but clinical trials have shown that they cause high blood pressure, diarrhea and adversely affect such organs as the lungs and the pancreas.
Blood cells circulate for months, while free hemoglobin is short-lived.
The modified hemoglobins seem to be running up against several million years of evolution, Klein said. Only earthworms use free hemoglobin to transport oxygen. Other animals package hemoglobin inside blood cells. Red blood cells don't just protect the body from hemoglobin, they also help hemoglobin do its job, Ho said. No scientist has ever been able to make a cell, so if hemoglobin is to be used in a blood substitute, the protein will have to be engineered to work without the help of a cell.