Elijah was a chubby baby and seemed perfectly healthy to his parents, Noami and Rich Patino of Woodbridge, N.J.

"I just thought, 'You know what? My son likes to eat,'" the boy's mother said Thursday on NBC's Megyn Kelly TODAY show.

But at the boy's five-month checkup, a physician thought his belly seemed unusually large, and recommended an ultrasound. It showed that his spleen was three times normal size.

That finding touched off a years-long journey for answers, leading them finally to physicians at Children's Hospital of Philadelphia.

Neil Romberg, a pediatric immunologist at the hospital, joined Elijah, now 7, and his family on the TV show to explain what happened.

Elijah was born with an extra copy of a gene that regulates his immune system, causing his white blood cells to attack his own organs. His mother turned out to have an extra copy of the gene as well — making them the only two people known to have this abnormality, CHOP researchers say. (Noami Patino has experienced only mild, occasional symptoms because the disease seems to manifest differently in women.)

Physicians in New Jersey initially diagnosed Elijah with a type of anemia, causing him to suffer fatigue and abdominal pain, and they kept his symptoms in check with a steroid called prednisone.

But in 2013, at age 2½, he became much sicker. His skin turned yellow, and his lungs and liver showed signs of infection. His parents sought answers from physicians in Ohio and New York before the genetic abnormality was discovered by Romberg and his colleagues.

"This gene is important for letting the different parts of the immune system talk to each other," Romberg said on the TV show. "My thought perhaps was if he had an extra copy of this gene, his immune system would be overly chatty."

The boy's symptoms now are kept in check with cyclosporine A, a powerful immune-suppressing drug that is typically used to prevent the rejection of transplanted organs, CHOP said. He also takes a daily antibiotic to help prevent infections.

The medicines do not represent a cure, but Romberg said they have been "remarkably effective," with apparent consequences for Elijah's older sister, Mya, 12.

"He's got enough energy to annoy his sister," the immunologist told the TODAY show hosts.

Romberg and his colleagues described Elijah's case in the June issue of the Journal of Allergy and Clinical Immunology.