Two years after being diagnosed with kidney failure, 23-year-old Kolls spread the word that she needed a kidney.
Alessio, 32, saw her message and made a life-altering decision.
RELATED: Cross-town rivals forever linked: White Sox fan donates kidney to Cubs fan
"I saw the post and just wanted to help her out... I just kind of went for it," Alessio said.
The kindness was almost too good for Kolls to believe.
"I did not believe it was for real up until the day of transplant," she said.
"I feel great, like better than ever," Kolls told ABC7 Tuesday morning.
Alessio said he was a little nervous going into surgery but feels "exactly the same" as he did before surgery.
Though they'll be cheering for different teams, both plan to be at Wrigley next year after Cubs player, Ian Happ invited them to the crosstown games.