Boy, 8, meets his bone marrow donor for the first time: 'Thank you for saving my life'

— -- An 8-year-old cancer survivor met the stranger who donated her bone marrow to him for the very first time on "Good Morning America" today.

“Thank you for saving my life,” A.J. told his donor, Alex Momany, during the emotional meeting.

“It was my pleasure,” Momany, 24, replied.

A.J., 8, was connected with Momany through the Be the Match Bone Marrow Registry.

Until this point, A.J.’s family and Momany had only communicated on the phone, but now they’re “looking forward to sharing life with her” A.J.’s mom, Alexa, said on “GMA.”

“It’s overwhelming,” A.J.’s father, Jeff, said while fighting back tears. “We’ve thought a lot about meeting you and getting to know you more, and we’re so excited and so thankful.”

A.J.’s battle against leukemia

A.J. was diagnosed with leukemia on his 4th birthday, Alexa told ABC News. The family asked to only be identified by their first names for privacy reasons.

"He was a really healthy boy and ... all of the sudden, in the middle of the night, he woke up with leg pain," Alexa said. "The following day the leg pain was so excruciating that we had to take him to the ER."

Alexa added that in 2013, when they found out it was leukemia, the family was living overseas because A.J.'s father, Jeff, was serving in the military. They came back to the U.S. for treatment following the diagnosis.

"A.J. was so sick. The treatment was so intense," Jeff told ABC News. "Alexa and I were shifting each other out at the hospital."

A.J. eventually went into remission, his mother said. But in the summer of 2015, he relapsed.

"It came back in two locations," Jeff said of his son's cancer. "That's a clear indication that the ... treatment wasn't working."

Alexa added that this was when they knew "that a transplant was really his only option."

"The first step was for all of us to be tested and ... none of us were a match," she added of her, her husband, and her two older children, Sophia and Nate.

The mother said she then turned to Be the Match, a national bone marrow donation organization that helps patients connect with lifesaving donors. To date, Be the Match has facilitated more than 80,000 marrow and core blood transplants.

Through the organization, A.J. was matched with Momany, then a 22-year-old college student from Ohio, who offered to donate her bone marrow to save his life. At the time, the family only knew her age and that she was a woman.

"It felt like a very long road leading up to his transplant day," Alexa said. "He went through a lot of heavy-duty chemotherapy."

Despite going through the grueling treatments, parents say A.J. was still a typical kid, especially enjoying dancing and playing with his toys.

Alexa said she wrote an emotional Facebook post to her son’s anonymous donor on the day of her surgery: "I'm thinking of you as you head to the hospital today for your procedure, and I thank you from the very depths of my heart for what you've done for my little boy."

Several days after the surgery, unbeknownst to A.J.’s family, Momany had also written a letter explaining why she chose to donate.

“This boy may somebody be someone’s husband, someone’s father, grandfather, son-in-law,” she wrote of her donor recipient. “Maybe he will take the world by storm and find the cure for cancer … or maybe he won’t. The point is, he is important.”

The transplant ended up being successful, and after 270 days in the hospital, A.J. found out he was cancer-free.

"Saying thank you never really feels like enough," Alexa said. "When someone saves your child's life."

Momany, who is getting married on Saturday, invited A.J. and his family to her wedding.

Today, Momany shared her message for anyone contemplating becoming a donor: “It’s the right thing to do.”

“Don’t be afraid to donate. Don’t be afraid to join,” she said. “You’re never going to regret saving someone. There’s nothing bad that can come out of donating. It’s all positives.”