<p>A compartmental model depicting tau and antibody (IgG) levels in brain, interstitial fluid (ISF), cerebrospinal fluid (CSF) and plasma. CSF tau is truncated with levels of ~1 nM, while tau in ISF exists as a full-length molecule with levels of 3–5 nM (4-5x greater than in CSF) [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125614#pone.0125614.ref047" target="_blank">47</a>]. Concentrations of p-tau are estimated to be about 1–10% of total tau levels. Tau antibody concentrations are 1–3 nM in CSF and ISF [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125614#pone.0125614.ref044" target="_blank">44</a>]. Antibody engagement of p-tau in CSF and ISF would enable clearance of tau via a variety of antibody-mediated mechanisms. Full-length tau is indicated as a molecule containing N-terminal (orange line), mid-domain (blue line), microtubule-binding repeat region (black box), and C-terminal (grey line) regions, whereas truncated tau in the CSF compartment is indicated by the mid-domain and N-terminal fragments.</p
