Tau protein is was found to be elevated in the cerebrospinal fluid (CSF) of patients with Alzheimer disease (AD), reflecting neurofibrillary tangle formation. CSF tau concentrations, when combined with age in a discriminant function analysis, increased the power to maker a correct diagnosis of AD in subjects less than 70 years old. Interleukin 6 (IL-6) contributes to the interaction between immune mechanisms and brain. CSF concentrations from geriatric patients with major depression showed significantly reduced levels of both IL-6 and the soluble Il-6 receptor (sIL-6R), measured by enzyme-linked immunosorbant assays (ELISA), compared with control CSF.