The majority of existing embryonic stem cell lines may be worthless for therapeutic purposes according to new research carried out by scientists in the US. As reported in the scientific journal Nature, when the researchers added embryonic stem cells to serum from human blood, antibodies attached themselves to the cells, suggesting that if they were transplanted into a human body, the immune system would reject them. The team of Californian scientists believes that this reaction is the result of the method by which most stem cell lines have been grown and maintained in the laboratory. Stem cell lines are grown in a culture containing nutrient broth and other feeder cells. The feeder cells in question are typically embryonic stem cells from mice, and the nutrient broth itself often contains animal serum. These mouse cells have a particular molecule on their surface which triggers the human immune system. Eating red meat and dairy products has helped sensitise people to the molecule, says the team. Doubts have previously been raised about the wisdom of growing human embryonic stem cells in animal-derived substances. 'Now we've identified an actual reason for being concerned,' says team member Fred Gage, from the Salk Institute for Biological Studies in La Jolla, US. 'We're not saying that all available lines should be thrown out. [But] we need to take caution when using these cells as therapeutics,' Professor Gage continued. However, while current stem cell lines may now prove to have little clinical value, they can still be put to good use for basic research purposes, the scientists believe.