MONTREAL, Feb. 17 (UPI) -- Canadian researchers said Friday they have uncovered a natural defense mechanism to Alzheimer's disease. Not surprisingly, it involves stem cells -- those derived from bone marrow. In Alzheimer's patients, plaque forms in the brain, but the brain's resident immune cells, called microglia, can't fight off the substance. The plaque can then kill off the brain's neurons, or nerve cells. However, microglia harvested from bone marrow stem cells do appear capable of defeating the plague, said researchers from the Faculty of Medicine at Université Laval and the research centre at Centre hospitalier universitaire de Québec, Canada....