Dr. Joseph Wu and his team tested a theory that embryonic stem cells are biologically protected from the human body's immune system because they are responsible for forming tissue at the early stages of life.
But in a study with mice, Wu found that once stem cells were transplanted, they did trigger a reaction by the immune system.
The results suggest that doctors would have to introduce anti-rejection drugs, making potential stem cell treatments more complicated.
"I think the general public has this unrealistic expectation that human cells are ready to go and, voila, we're going to cure heart disease, we're going to cure Alzheimers, we're going to cure Parkinsons," Wu said. "I think it just points out one of the hurdles."
The tests were performed on mice because of cancer risks associated with transplanting unmodified embryonic stem cells.
The results are being published by the National Academy of Sciences.