Autoantibodies (AB) against N-methyl-D-aspartate-receptor subunit-NR1 (NMDAR1) are highly seroprevalent in health and disease. Symptomatic relevance may arise upon compromised blood-brain-barrier (BBB). However, it remained unknown whether circulating NMDAR1-AB appear in the cerebrospinal fluid (CSF). Of N=271 subjects with CSF-serum-pairs, 26 were NMDAR1-AB seropositive, but only 1 was CSF-positive. Contrariwise, tetanus-AB (non-brain-binding) were present in serum and CSF of all subjects, with CSF levels higher upon BBB-dysfunction. Translational mouse experiments proved the hypothesis that the brain acts as ′immunoprecipitator′: Simultaneous injection of NMDAR1-AB and the non-brain-binding GFP-AB resulted in high detectability of the former in brain and the latter in CSF. This article is protected by copyright. All rights reserved.