A highly selective filter between your bloodstream and your brain that keeps most drugs — and most peptides — out of the CNS.Formed by tight junctions between brain endothelial cells. Most peptides cannot cross the BBB due to size and polarity. Semax and selank are among the exceptions, attributed to specific structural features. Some neuroprotective peptides (NAP, humanin) have been studied with intranasal delivery as a way to bypass the BBB via olfactory nerve pathways, though the intranasal-to-brain transport efficiency for most peptides remains uncertain.