Neuroinflammation and Cognitive Decline in Alzheimer Disease

Inclusion Criteria: * Informed consent * Age more than 50 years (included) * necessary knowledge of French (write and oral) to do neuropsychological tests * Study level upper (or equal) than 7 years (considering first year of grammar-school as start) * People with Alzheimer Disease defined as National Institute of Neurological and Communicative Disorders and Stroke (NINCDS) and the Alzheimer's Disease and Related Disorders Association (ADRDA) standards : Light to mild AD defined by Mini-Mental State Examination (MMSE) score between 15 and 25 (included) * Social security affiliation. Exclusion Criteria: * MMSE score lower than 15 and upper or equal to 26 * Evolutive disease which could possibly had consequences on central nervous system * Inflammatory disease or evolutive neoplasia and/or C reactive protein (CRP) upper than 10mg/L * Chronic use of alchohol and/or drug * Serious depression defined by Montgomery Asberg Depression Rating Scale (MADRS) score higher than 18 * Surgical or medical condition in the last 3 months * Long term treatment which could possibly interfere with inflammatory process (especially the month before PET \[18F\]DPA-714 imaging). * Treatment by N-Methyl-D-Aspartate antagonist * Treatment by Minocycline * Treatment by benzodiazepine (especially the month before PET \[18F\]DPA-714 imaging) (Zolpidem, zopiclone and loprazolam excepted) * Anomaly at neurological examination which is not a classical symptom * Contraindication to magnetic resonance imaging (RMI) * Florbetapir\[18F\] hypersensibility * Participation to an other experimental protocol with drug. * people under guardianship