AMAZING STORIES OF SACRIFICE AND SURVIVAL AIRING ON "20/20," FRIDAY, DECEMBER 21

From a woman's remarkable will to survive after being crushed by a car to an unlikely friendship between a morbidly obese man and the physical trainer who helped him lose over 400 pounds, "20/20" reports on unbelievable stories of sacrifice, survival and generosity this holiday season. "The Greatest Gift" airs on "20/20," FRIDAY, DECEMBER 21 (10:00-11:00 p.m., ET) on the ABC Television Network. Reports include:

Give a life to get a life. Within hours of Katie Trebing's birth Dec. 12, 2002, she needed a blood transfusion to save her life � the first of many to come. Steve and Stacy Trebing's daughter was born with Diamond Blackfan Anemia, a rare bone marrow disease that affects just 30 out of four million births each year in North America. Katie's body wasn't making any red blood cells to carry oxygen to her organs, and never would. She needed transfusions every three to four weeks or she would die. But that treatment came with devastating side effects, drastically shortening Katie's life span. There was just one way to cure Katie � a bone marrow transplant from a perfectly matched sibling. Yet her older brother, Calvin, was not a match. So the Trebings were presented with a process of testing embryos called pre-implantation genetic diagnosis. The outcome would ensure their next child and Katie shared the same bone marrow DNA. But the bone marrow transplant was a perilous operation: It would either cure their daughter or kill her. Elizabeth Vargas reports.

Losing weight and gaining not only self esteem, but a best friend. After spending nearly a decade on the couch in his parent's house eating pizza, drinking soda and playing video games, David Smith lived as a recluse. He weighed 635 pounds, had no support system or close friends and was ready to kill himself � unable to withstand the pain of being mocked and taunted because of his size. Smith was full of despair and had no hope, until he saw trainer and lifestyle correspondent Chris Powell on the television station KTVK in Phoenix. Smith e-mailed him and asked for help. Surprisingly, Powell came out to meet Smith and together they embarked on the journey of a lifetime. Powell, without any financial compensation, began working with Smith, changing his eating habits and starting him on an exercise routine. After just 26 months, Smith lost 401 pounds. But Smith's transformation is ongoing. No longer morbidly obese and imprisoned by social anxiety, David is working at a gym and studying to become a personal trainer. David Smith now has a bright future and a best friend. Chris Powell, he says, is his angel on earth. Chris Connelly reports.

Total body reconstruction: life after 200 surgeries. When she was four years old, Heather Bland was leaning against a door in her mother's car as she backed out of a parking space. The door opened and Bland fell out, and was crushed by the front wheel. Bland was not supposed to make it through the night. She did. She was not supposed to walk. She did. She was not supposed to be able to have a child. She did. Now, 35 years after the accident, with some 200 surgeries behind her, Bland still battles serious health issues, but her amazing spirit and optimism help her with the fight. Jay Schadler reports.

Giving the greatest gift to children and teachers this holiday season. Teachers spend more than $1 billion a year on supplies for their classrooms, and yet they still do not have everything their classrooms need. A website called DonorsChoose.org brings help to classrooms around the country that require anything from the most basic supplies to the most innovative teaching tools. From pencils to computers to field trips, teachers post their unique requests on the site and donors can select which proposal they want to fulfill. Since founder Charles Best started the nonprofit seven years ago, more than $15 million has been funded. Not only are teachers getting the essentials that they need, but donors are fulfilled as well. Bob Brown reports.

"20/20" is anchored by Elizabeth Vargas and John Stossel. David Sloan is executive producer.