Kelsey Rae Zwick could not contain her emotions when a compassionate stranger in first class insisted on switching seats with her – and though she did not get to thank him properly on the plane, she finally got her chance on social media after her Facebook post went viral.

Zwick had been flying from Orlando to Philadelphia with her 11-month-old daughter Lucy earlier this week. Since the baby suffers from chronic lung disease, the two of them were heading to the Children's Hospital of Philadelphia (CHOP) for treatment.

"I was pushing a stroller, had a diaper bag on my arm and also lugging an oxygen machine for my daughter," wrote Zwick in an open Facebook letter to the passenger. "We had smiles on our faces as we were headed to see her ‘friends' at CHOP."

As they settled into their pre-boarded seats, however, Zwick was stunned to be approached by a flight attendant who said that a man in first class wanted to switch seats with her.

"Not able to hold back tears, I cried my way up the aisle while my daughter Lucy laughed!" wrote Zwick. "She felt it in her bones too… real, pure, goodness. I smiled and thanked you as we switched but didn't get to thank you properly.

"Sooo… thank you. Not just for the seat itself but for noticing. For seeing us and realizing that maybe things are not always easy. For deciding you wanted to show a random act of kindness to US. It reminded me how much good there is in this world.

"I can't wait to tell Lucy someday. In the meantime… we will pay it forward."

Zwick's Facebook letter was shared hundreds of thousands of times until American Airlines finally tracked down the mystery flier: 46-year-old Jason Kunselman.

Kunselman, who is an Executive Platinum frequent flier, says that he himself was brought to tears when he saw Zwick and Lucy boarding the plane with all of their medical equipment in tow.

"I expected to see her in the front since she had the oxygen machine with her but when I boarded she was in the back," Kunselman told The Daily Mail. "So I asked the flight attendant if she thought the young lady with the baby would be more comfortable in my seat."

Kunselman says that he did not think much of the good deed until his wife saw Zwick's Facebook post days later. American Airlines has since connected the two passengers on social media so they could express their appreciation for each other, and the airline says that they soon plan on formally thanking Kunselman for his generosity.

Be Sure And Share This Sweet Story Of Kindness With Your Friends on Social Media – Feature photo by Kelsey Rae Zwick / Jason Kunselman