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The blood-brain barrier prevents many of the chemicals in the bloodstream from reaching the brain. This usually means that viruses and bacteria in the bloodstream cannot reach the brain, however, in diseases like bacterial meningitis or HIV encephalitis, this barrier is breached. The blood-brain barrier is also affected by multiple sclerosis, Alzheimer’s disease, and some brain tumors.
In bacterial meningitis, the bacteria and the body’s inflammatory response can cause a partial breakdown of the blood-brain barrier. This allows the bacteria to enter the brain, where it can cause some of the neurological symptoms of meningitis such as headache and seizures. This degradation of the barrier also allows antibiotics to reach the brain more easily, which can make treatment more effective.
Like bacterial meningitis, the HIV virus can also sometimes breach the blood-brain barrier, causing HIV encephalitis, an inflammation of the brain. In patients with compromised immune systems, other viruses may also cross into the brain and cause disorders such as progressive multifocal leukoencephalopathy (PML). PML, which is caused by inflammation of the white matter of the brain, is a very serious and sometimes fatal disease.
Brain tumors can also affect this barrier. Often, the blood vessels in a tumor have a poorly developed barrier, allowing some chemicals from the blood to enter the brain. A more aggressive tumor will do more damage to this barrier than a benign tumor.
Multiple sclerosis is another disease that affects the blood-brain barrier. In multiple sclerosis, the immune system attacks myelin, which coats and protects nerve and brain cells. In some places, the immune system is able to cross the blood-brain barrier and attack the cells in the brain.
Some evidence shows weakening of the blood-brain barrier in patients with Alzheimer’s disease. This degradation allows chemicals from the bloodstream to enter the brain, which may cause the death of some types of brain cells. This can cause or exacerbate the symptoms of Alzheimer’s disease.
De Vivo disease is another disease of the barrier between blood and brain. Unlike many other diseases, which make it too easy for chemicals to enter the brain, De Vivo disease prevents necessary compounds from crossing over into the brain. De Vivo disease is a genetic defect in a type of cell that transports glucose across the blood-brain barrier. This means the brain does not get enough energy to function normally, and can lead to neurological problems such as mental retardation.
I just want to add that scientists once believed that blood brain barrier development did not begin until about six weeks after birth. Astrocyte is a brain cell that they thought didn't begin development until after birth.
But researchers have proven that pericytes are actually the brain cells that are required for our blood brain barrier development and that they are present in our fetal brains well before birth.