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Circumventricular organs CVOs are individual structures located adjacent to the fourth ventricle or third ventricle in the brain, and are characterized by dense capillary beds with permeable endothelial cells unlike those of the blood-brain barrier. Such zones exist at the border of the area postrema— nucleus tractus solitarii NTS ,  and median eminence— hypothalamic arcuate nucleus.
In its neuroprotective role, the blood-brain barrier functions to hinder the delivery of many potentially important diagnostic and therapeutic agents to the brain. Therapeutic molecules and antibodies that might otherwise be effective in diagnosis and therapy do not cross the BBB in adequate amounts. Mechanisms for drug targeting in the brain involve going either "through" or "behind" the BBB. Modalities for drug delivery to the brain in unit doses through the BBB entail its disruption by osmotic means, or biochemically by the use of vasoactive substances such as bradykinin ,  or even by localized exposure to high-intensity focused ultrasound HIFU.
However, vectors targeting BBB transporters, such as the transferrin receptor , have been found to remain entrapped in brain endothelial cells of capillaries, instead of being ferried across the BBB into the targeted area. Meningitis is most commonly caused by infections with various pathogens , examples of which are Streptococcus pneumoniae and Haemophilus influenzae.
When the meninges are inflamed, the blood-brain barrier may be disrupted. Antibiotics used to treat meningitis may aggravate the inflammatory response of the central nervous system by releasing neurotoxins from the cell walls of bacteria-like lipopolysaccharide LPS.
A brain abscess is a rare, life-threatening condition. Local sources may include infections of the ear, the oral cavity and teeth, the paranasal sinuses, or an epidural abscess. Remote sources may include infections in the lung, heart or kidney. A brain abscess may also be caused by head trauma or as a complication of surgery. In children cerebral abscesses are usually linked to congenital heart disease. Normally, a nervous system would be inaccessible to the white blood cells due to the blood—brain barrier.
However, magnetic resonance imaging has shown that when a person is undergoing an MS "attack," the blood—brain barrier has become more permeable in a section of the brain or spinal cord, allowing white blood cells called T lymphocytes to cross over and attack the myelin.
One consideration is multiple sclerosis may be a disease of the blood—brain barrier. People with neuromyelitis optica have high levels of antibodies against a protein called aquaporin 4 a component of the astrocytic foot processes in the blood—brain barrier. Activated macrophages release virions into the brain tissue proximate to brain microvessels. These viral particles likely attract the attention of sentinel brain microglia and perivascular macrophages initiating an inflammatory cascade that may cause a series of intracellular signaling in brain microvascular endothelial cells and damage the functional and structural integrity of the BBB.
He found then the brains did become dyed, but the rest of the body did not, demonstrating the existence of a compartmentalization between the two. At that time, it was thought that the blood vessels themselves were responsible for the barrier, since no obvious membrane could be found.
The concept of the blood—brain barrier then termed hematoencephalic barrier was proposed by a Berlin physician, Lewandowsky, in
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