12:35 PM PT -- Presley's family tells us the 15-year-old died Friday morning ... just 2 days after the call with Taylor.
Taylor Kinney is helping one young fan in her battle with cancer ... lifting her spirits with a very special FaceTime call from the set of her favorite TV show, "Chicago Fire."
Presley Long is one of Taylor's biggest fans -- she's never missed an episode of Taylor's hit show -- but she's just 15 years old and battling a very aggressive form of cancer.
So, you can imagine the boost to her system when she got a surprise FaceTime call from Taylor this week ... and the video from their intimate chat is super emotional.
Presley, who suffers from a gastrointestinal neuroectodermal tumor (or GNET), got in touch with her favorite celeb thanks to her mom's friend and the producers at "Chicago Fire." Her mom dialed up an old pal who runs a cancer foundation -- the Nikolas Ritschel Foundation -- and they put her in touch with the show's producers to set up the call.
As you can see in the video ... Taylor gives Presley the inside scoop on the day's filming, then tells her the cast and crew are keeping her in their thoughts and prayers. Taylor's even pulling strings to get Presley on set!!!
Taylor left quite an impression on Presley and her fam ... her mother was crying tears of joy throughout the convo.
Presley's also a huge fan of Ellen DeGeneres ... and we're told she's hoping to meet Ellen too.