<qa>  What is the central nervous system harvested at the time of death?This is a longitudinal cohort study in which participants will be seen approximately annually at the NIH Clinical Center. Most visits will extend over several days. Participants will receive ongoing care by their outside clinician. They may also concurrently participate in additional research protocols at the NIH or elsewhere, and data may be shared between those protocols and the current one. At the time of death, the central nervous system CNS brain, spinal cord, retinas, and cerebrospinal fluid, as well as lymph nodes and possibly other lymphoid tissue, will be harvested. 