Amyloid and Tau Explained

A plain-language guide to amyloid plaques and tau tangles—the two proteins at the heart of Alzheimer's disease—how they damage brain cells, how doctors measure them, and why they matter for new treatments.

Last reviewed: October 2026

If you've read about Alzheimer's disease, you've probably seen two words again and again: amyloid and tau. They are proteins that occur naturally in the brain. In Alzheimer's, both start to misbehave, building up into the "plaques" and "tangles" that are the hallmarks of the disease. This article explains what they are, what they do, and why they matter so much to doctors and researchers today.

A quick look at healthy brain cells

The brain contains tens of billions of nerve cells called neurons. They pass messages to one another across tiny gaps called synapses. According to the National Institute on Aging (NIA), a single neuron may have as many as 7,000 connections with other neurons.

Alzheimer's disrupts this communication. Over time, many neurons stop working properly, lose their connections and eventually die. The NIA notes that early loss of synaptic connections is one of the main hallmarks of the thinking decline in Alzheimer's.

Two proteins are central to that damage.

Amyloid: the plaques between cells

Beta-amyloid is a small protein fragment cut from a larger protein called the amyloid precursor protein. It comes in several forms. The NIA says one form, beta-amyloid 42, is thought to be especially toxic.

In Alzheimer's, abnormal levels of beta-amyloid clump together between neurons to form amyloid plaques. These plaques disrupt how cells work.

Beta-amyloid can also build up in the walls of blood vessels in the brain, one of several vascular problems the NIA describes in people with dementia.

Tau: the tangles inside cells

Tau works inside neurons. Think of each neuron as having a system of tiny train tracks, called microtubules, that carry nutrients and other supplies from one end of the cell to the other. In healthy cells, tau helps hold those tracks steady.

In Alzheimer's, the NIA explains, abnormal chemical changes cause tau to let go of the tracks and stick to other tau molecules. These form threads that join into neurofibrillary tangles. The tangles block the cell's transport system, which harms communication between neurons.

Tau tangles are not unique to Alzheimer's. The NIA notes that tau forms tangles in many other dementias, too—for example, some forms of Frontotemporal Dementia.

How amyloid and tau work together

Scientists are still working out the full story, but the picture is getting clearer. According to the NIA:

Abnormal tau builds up in specific brain regions involved in memory.
Beta-amyloid clumps into plaques between neurons.
When beta-amyloid reaches a tipping point, tau spreads rapidly throughout the brain.

Damage usually begins in memory areas such as the entorhinal cortex and the The Hippocampus, which is why memory problems are often the first sign. Later, areas controlling language, reasoning and social behavior are affected.

Amyloid and tau are not the only players. The NIA also describes chronic inflammation from brain support cells called microglia that fail to clear away debris, and blood vessel problems that reduce blood flow and weaken the barrier protecting the brain. See Inflammation and the Brain.

Important: having amyloid doesn't always mean having symptoms

Here is a point that surprises many people. The NIA notes that it's possible to have amyloid plaques and never develop the symptoms of Alzheimer's. Doctors therefore look at amyloid results alongside memory tests, medical history and other findings. An amyloid scan showing few or no plaques usually means Alzheimer's is not the cause of a person's symptoms.

Brain changes from Alzheimer's can begin before any symptoms appear. In 2024, an Alzheimer's Association workgroup published revised criteria that define Alzheimer's as a biological process that begins with these brain changes while people still have no symptoms. Symptoms appear later as the changes build up. The authors noted that the criteria are general principles reflecting current science, not step-by-step instructions for doctors, and that other conditions, cognitive reserve and individual resistance can affect how biology and symptoms line up.

How doctors measure amyloid and tau

Before the early 2000s, the only sure way to confirm Alzheimer's was to examine the brain after death. Now, according to the NIA, biomarkers—measurable signs in the body—can show amyloid and tau in living people:

Amyloid PET scans detect amyloid plaques in the brain.
Tau PET scans detect tau tangles. The NIA says these are mostly used in research rather than everyday care.
Spinal fluid (CSF) tests, taken by lumbar puncture, measure beta-amyloid 42, tau and phospho-tau. See Spinal Fluid (CSF) Testing.
Blood tests measure brain proteins in the blood. The 2024 criteria highlight blood tests for phosphorylated tau 217 (p-tau217) as one of the early-changing biomarkers. See Blood Tests for Alzheimer's.

The NIA stresses that no single test can diagnose dementia. Biomarkers are one part of a complete evaluation. Some tests are only available through specialty clinics, and insurance may cover only certain ones. Learn more in Biomarkers and Brain Imaging: MRI, CT and PET.

Why this matters for treatment

For decades, researchers have tried to remove or block amyloid. Two anti-amyloid drugs, lecanemab (Leqembi) and donanemab (Kisunla), have received full approval from the U.S. Food and Drug Administration for early Alzheimer's. The NIA says they reduce amyloid plaques and can slow cognitive decline in some people. They are not cures, and they carry risks, including brain swelling or bleeding known as ARIA. See Lecanemab (Leqembi), Donanemab (Kisunla) and ARIA: Side Effects of Anti-Amyloid Drugs.

Because these drugs target amyloid, doctors confirm that amyloid is present—usually with a PET scan or spinal fluid test—before prescribing them.

Researchers are also testing treatments aimed at tau, inflammation and other targets. See Dementia Research Today and Joining a Clinical Trial.

The bottom line

Amyloid forms plaques between brain cells.
Tau forms tangles inside brain cells.
Together with inflammation and blood vessel changes, they damage and eventually kill neurons.
These changes can start before symptoms appear, and having amyloid alone doesn't guarantee symptoms will develop.

When to see a doctor

If you or someone close to you is having memory or thinking changes that affect daily life, talk with a doctor. Amyloid and tau testing isn't right for everyone, and results need careful explanation. A doctor or memory specialist can help decide which tests, if any, make sense for you. See Getting a Diagnosis.

Sources

Not medical advice. This article is general information, not a diagnosis or treatment plan for any individual. Talk with a doctor about your situation.

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