Overview:Scientists believe they have cracked the essential mystery of the malignant cell and the first targeted therapies have been developed. But very quickly cancer reveals new layers of complexity and a formidable array of unforeseen defenses. In the disappointment that follows, many call for a new focus on prevention and early detection as the most promising fronts in the war on cancer. But other scientists are undeterred, and by the second decade of the 2000s their work pays off. Perhaps most exciting of all is the prospect of harnessing the human immune system to defeat cancer. Doug Rogers, a 60-year-old NASCAR mechanic with melanoma, and Emily Whitehead, a six-year-old child afflicted with leukemia are both pioneers in new immunotherapy treatments.