Your blood-brain barrier (BBB) is a tightly locked layer of cells that defend your brain from harmful substances, germs and other things that could cause damage. It’s a key part of maintaining your brain health. It also holds good things inside your brain, maintaining the organ’s delicate chemical balance.
Though the name includes the word “barrier,” it’s really more of a filter and behaves like a gatekeeper to your brain. It’s there to keep harmful things out and hold helpful things in. It also controls how various chemical molecules (including compounds your body needs or makes itself) enter and exit your brain.
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Inside your blood vessels is a layer of specialized cells called the endothelium. But inside your brain, the endothelium is different.
Endothelial cells lining the inside of your brain’s blood vessels are tightly packed together, forming your blood-brain barrier. They’re so tightly packed that there’s almost no space for anything to slip through without help. These cells have a lipid-based outer membrane.
Some things can get through your BBB if they’re small enough. Others can get through because they’re lipid-soluble. That means they can pass through your blood-brain barrier without it repelling them. Larger or water-soluble molecules can’t get through the BBB on their own.
Large molecules can’t slip between the interlocking endothelial cells because of their size. Water-soluble molecules can’t easily pass through your BBB because its cell membranes are lipid-based, which repels water-soluble molecules. If large or water-soluble molecules — including nutrients — need to get through, they need transportation to help them across.
Some examples of drugs and substances that can get through the BBB (either on their own or with transport help) include:
The above list mainly refers broadly to medication classes that can pass through your BBB. But medications from many more classes can also make it through. The list of what can make it through your BBB is extremely lengthy, so medications that fall into the above classes make up only a fraction of the medications that can cross your BBB.
Right now, the best methods to find or predict which compounds can pass through your BBB involve highly complex algorithms and computer programming. So far, these methods list nearly 5,000 chemical molecules (including medications) that can (or should be able to) pass through your BBB.
Pathogens, such as bacteria or viruses, and many toxic substances generally can’t get through your BBB.
Your blood-brain barrier is highly secure, but it isn’t perfect. Inflammation can weaken it. Other conditions can also make it less effective. That can let harmful substances or pathogens into your brain (it can also make some medications, such as penicillin-type antibiotics, more effective at treating infections in your brain).
Conditions that involve any weakening in your BBB’s integrity include both acute and chronic illnesses.
Acute conditions are issues that are happening right now or very recently. Examples of acute conditions that can affect your BBB include (but aren’t limited to):
Chronic conditions are long-term concerns. They can last months (at minimum), but many — if not most — are permanent and can last for years. These include (but aren’t limited to):
Experts suspect many other chronic conditions can affect your BBB, but more research is necessary to confirm this.
For now, there’s no clear way to identify problems with your BBB. Some lab tests may be able to indirectly hint at BBB issues, especially tests on cerebrospinal fluid. Most conditions that involve BBB disruptions become apparent through symptoms that aren’t related to your barrier.
Experts are working on ways to weaken the BBB temporarily using magnetic resonance imaging-guided focused ultrasound. That may allow molecules inside of your brain to escape into your blood, making them easier to detect. Those molecules might be the key to detecting and diagnosing conditions like brain cancer, Alzheimer’s disease and Parkinson’s disease earlier. It might also allow some medications to make it through and treat conditions like cancer inside the brain. However, this research is still in its early stages, and it will take more research to determine if this is possible.
For now, there aren’t any treatments that target your blood-brain barrier directly. Instead, treatments target conditions that could disrupt your BBB. Some examples include:
There’s no direct way to prevent disruptions that affect your BBB. Instead, the best approach is to avoid conditions or circumstances that could lead to its disruption. Some things you can do include:
Your BBB is a key consideration in treating many conditions in your brain and throughout your body. That’s because many treatments — especially medications — can indirectly affect your brain if they can get through your BBB.
Other times, healthcare providers have to try to work around your BBB. That’s because it’s sometimes too good at its job. Medications that could treat infections or cancers in your brain may not be able to make it through your BBB.
To get around that, experts use medications specially formulated to avoid your blood-brain barrier’s filtering. They can use a “Trojan horse” approach, piggybacking medications on immune cells or other molecules that can get through your BBB.
Experts can also use your body’s natural chemistry to get around your BBB. An example of this is using the drug levodopa to treat Parkinson’s disease. Parkinson’s happens when there isn’t enough dopamine in your brain, which certain brain areas need for controlling muscle movements. Dopamine can’t get through your BBB, but levodopa can. Once it’s inside your brain, levodopa converts into dopamine, helping treat Parkinson’s symptoms.
Most sources credit other scientists, particularly Paul Ehrlich or Max Lewandowsky, with discovering the barrier in the late 1800s and early 1900s. However, the first use of the term “blood-brain barrier” was in a 1921 research paper authored by Lina Shtern (sometimes, spelled “Stern”) of Russia (in what is now Latvia) and Swiss physician Raymond Gautier.
Electron microscope images first captured in the 1960s and 1970s confirmed Shtern and Gautier’s work. Those images detailed the structure of the BBB in animal and human brains.
A note from Cleveland Clinic
Your blood-brain barrier (BBB) is a filtering layer of cells surrounding blood vessels in most areas of your brain. It’s a critical line of defense, keeping most harmful things out and most helpful things in. But it isn’t perfect. Many conditions can affect it, and healthcare providers have to consider the BBB when trying to treat all conditions, not just those that affect your brain.
While experts know much about the BBB, more research is necessary to understand all of how it works. Unlocking its inner workings can open the door to effective treatments for many brain-related conditions. That way, your BBB can become a barrier only to things that can harm you and not obstruct treatments that can help.
Last reviewed by a Cleveland Clinic medical professional on 04/17/2023.
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