Blood-brain barrier

The blood-brain barrier (BBB) is a selective permeability barrier formed by endothelial cells of cerebral capillaries, tightly joined by tight junctions, that separates circulating blood from the brain's extracellular fluid. It is not merely a passive filter but an active regulatory interface — a checkpoint that determines which molecules, cells, and signals can access the central nervous system.

The BBB's selectivity is dynamic. Inflammatory cytokines, chronic stress, and aging all increase its permeability, allowing peripheral immune signals to penetrate neural tissue. This "leaky" state is not a failure mode but a regulated transition: the brain opens its borders when it needs peripheral information, and seals them when the threat passes. The BBB is thus an active participant in the neuro-immune axis, not a passive obstacle to it.

The molecular transport mechanisms across the BBB — including the saturable transporters for interleukin-1β and other immune signals — are themselves regulated by neural activity. This means the barrier is not a static wall but a neurovascular gate: a control point whose openness is modulated by the brain's own state.