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How the blood–brain barrier protects (and sometimes fails) The blood–brain barrier (BBB) is a tightly regulated gateway separating the brain from circulating blood. It’s made of specialized endothelial cells, pericytes, and astrocytes that form the neurovascular unit, working together to decide what gets in and out. 1️⃣ In a healthy state The BBB allows essential nutrients—like glucose, amino acids, and oxygen—to enter while keeping out toxins, pathogens, and immune cells. Tight junction proteins seal the spaces between cells, and astrocytes help regulate water and ion balance. Microglia patrol for signs of trouble while maintaining calm, anti-inflammatory signaling. 🟢 Example: brain-derived neurotrophic factor (BDNF) moves across the BBB to support learning and repair. 2️⃣ When the barrier breaks down Inflammatory cytokines (like TNF-α, IL-1β, and IL-6) loosen those tight junctions, allowing immune cells and harmful molecules to leak into brain tissue. This triggers oxidative stress and microglial activation, which can damage neurons and accelerate disease. 🟢 Example: chronic stress, infection, or neurodegenerative conditions like Alzheimer’s can disrupt the BBB, leading to inflammation and cognitive decline. The BBB acts like the brain’s border patrol—permeable enough to supply fuel, but selective enough to preserve neural stability. When its integrity falters, even small leaks can have outsized effects on brain function and mental health.