amyloid plaque

An amyloid plaque is a clump of protein that collects in the spaces between nerve cells in the brain. The protein is beta-amyloid, which forms when a larger protein called the amyloid precursor is broken down. Beta-amyloid comes in several molecular forms, and the US National Institute on Aging notes that the form called beta-amyloid 42 is thought to be especially toxic. In the brain of a person with Alzheimer's disease, abnormal levels of this naturally occurring protein clump together into plaques that disrupt how the cells work.

Plaques are one of the hallmark signs of Alzheimer's, which is why tests for the disease look for them. They can be seen with an amyloid PET brain scan, inferred from a sample of spinal fluid, and, since the US Food and Drug Administration cleared the first such device in May 2025, estimated from a blood test. The FDA notes that plaques can also occur in other diseases, so finding them is one piece of evidence that a doctor reads alongside others, not a diagnosis on its own.

Some newer Alzheimer's drugs target beta-amyloid directly. The National Institute on Aging reports that two of them have full FDA approval for early Alzheimer's, that they help reduce amyloid plaques, and that studies have shown they can slow the decline in memory and thinking in some people.