An answer to a plea posted on Facebook by the mother of a 3-year-old girl with a rare metabolic disease has given the toddler a chance to survive, thanks to a liver transplant facilitated through social media.

Kennedy McCoy had been diagnosed with a rare metabolic disorder called S-adenosylhomocysteine hydrolase deficiency, a condition so rare it has only been identified in eight people in the world, of whom only six are now living.

Her mother, Donya McCoy, became aware there was a problem when Kennedy suffered a seizure when she was just 8 months old.

The disease, caused by a genetic mutation involving her liver, prevents Kennedy's body from processing protein.

Doctors were at first stumped as to what might be wrong with the girl, until Dr. S. Harvey Mudd, a retired 86-year-old metabolic specialist, agreed to come out of retirement to help with a diagnosis.

Once the extremely rare condition was diagnosed, doctors determined a liver transplant would be her best chance of survival.

She is now the first of that small group with the genetic condition to receive a liver transplant in hopes of combating the condition.

Her new liver was donated by Michael Thompson, a high school classmate of Kennedy's mother Donya McCoy, who posted a plea for a donor on Facebook.

Thompson replied with a message saying he would like to help.

"I think I asked him five or six times if he was serious, if that was a serious response," McCoy says of Thompson's message on Facebook.

Thompson said he was serious, and testing showed he was a match and was qualified as a liver donor.

"One of the greatest things I will probably do in my life, give more life to another human," Thompson, who donated 25 percent of his liver, wrote on his Facebook page in October. "Thank you, Donya McCoy, for letting me."

Kennedy's 8-hour surgery Tuesday at Children's Hospital of Pittsburgh involved four surgeons, two anesthesiologists and two nurses.

Kennedy would remain in the hospital for several months as she recovers, according to Donya McCoy, but her daughter is "tough," she said.

Thompson would remain in hospital recovery for about a week, said doctors who removed part of his liver for the transplant.

Donya McCoy expressed her gratitude for his actions.

"It's pretty special to think that part of him lives in Kennedy now," she said.

"I think the only way I could ever do what he has done is to teach Kennedy about the gift he gave her and hope that she pays it forward in her life," she added.