A woman who underwent a full-face transplant in 2011 after she was mauled by a chimpanzee was back in the hospital on Wednesday after her body began to reject the transplanted tissue in her face, doctors said.

Charla Nash, 62, was participating in a research study designed to determine whether tissue transplant patients could be weaned off traditional anti-rejection drugs when her immune system began to attack her facial tissue, Brigham and Women’s Hospital in Boston said in a statement.

Ms. Nash was in no danger of losing her face, however, Dr. Bohdan Pomahac, the hospital’s director of plastic surgery transplantation, said in a statement. He said she was “experiencing a moderate rejection episode.”

“The viability of Charla’s face transplant is not in jeopardy,” he said. “Overall, she is doing well.”