“…Further, the missing or minimal ability of these proteins to cross cellular barriers without a carrier restricts their therapeutic use on well-exposed and accessible targets, as they cannot cross, for example, the blood–brain barrier (BBB) in sufficient amounts [ 32 ]. As a consequence, only roughly 0.1–0.4% of an individual IgG plasma concentration can be found in the CNS [ 33 , 34 , 35 ], and this can only be enhanced by using either trans-cellular transport mechanisms like transferrin- [ 36 ] or insulin receptor-mediated transcytosis [ 37 ], or by exploiting pharmaceutical particulate formulations [ 38 , 39 ]. On the contrary, antibody formats with a reduced size, such as Fab, single chain variable fragments (scFvs) or sdAbs can more efficiently diffuse into tumors and cross specific tissue barriers, e.g., the BBB, without losing their main properties, such as affinity and specificity for their antigen [ 30 , 40 , 41 ].…”