Wherever it's buried in the body, a disease leaves traces in the blood—or so the thinking goes. But finding these biomarkers, which can help catch the disease early on, has been an exercise in futility, with one promising candidate after another losing its luster once it receives scrutiny. A team of chemists and other researchers now propose a new way to pick up biomarkers with a blood test: by screening for antibodies that the body makes in response to particular diseases. So far, the group has reported results for only a small number of Alzheimer's disease patients. But they are...