The present invention relates to the treatment of brain or neurologic disorders. In this study, the inventors hypothesized that a Fc receptor (FcRn) expressed by endothelial cells from the BBB was responsible for this efflux. As a proof of concept, the inventors engineered a fragment antigen-binding (Fab) of trastuzumab and characterized it in preclinical models for translational therapeutic purpose. They demonstrated the safety and equal efficacy with trastuzumab in vitro, and in vivo using a patient-derived xenograft model of HER2 overexpressing breast cancer. They successfully engineered and thoroughly did the preclinical development of a trastuzumab Fab as effective as the native IgG, and capable of doubling brain penetration and significantly reducing brain-to-blood efflux after intra-cerebrospinal fluid injection. This Fab could thus be a new and original effective weapon in the treatment of HER2 breast cancer brain metastases, and thus used as a proof of concept of the powerful use of antigen binding fragment (antibody Fab derivatives) in brain or neurologic disorders. Thus, the present invention relates to an antigen binding fragment which binds to an antigen for use in the treatment of brain or neurologic disorders in a subject in need thereof.