Amyloid plaques are clumps of protein that build up between brain cells and are the hallmark feature of Alzheimer’s disease. They form when a small protein fragment called amyloid-beta, typically 38 to 43 amino acids long, misfolds and sticks together outside neurons. Over time, these sticky clusters harden into insoluble deposits that disrupt communication between brain cells and contribute to cognitive decline.
How Amyloid Plaques Form
Amyloid-beta doesn’t start as a problem. It’s a byproduct of a larger molecule called amyloid precursor protein (APP), which sits in the outer membrane of brain cells and plays a role in normal cell signaling. The trouble begins with how APP gets cut apart.
Two enzymes do the cutting. The first, called beta-secretase, snips APP at one site, leaving behind a fragment still anchored in the cell membrane. Then a second enzyme, gamma-secretase, makes a cut within the membrane itself, releasing the amyloid-beta fragment into the space between cells. Depending on exactly where gamma-secretase cuts, the resulting fragment is either 40 or 42 amino acids long. The 42-amino-acid version is especially prone to clumping and is considered the more toxic form.
There’s actually a harmless alternative. If a different enzyme (alpha-secretase) makes the first cut instead of beta-secretase, it slices right through the middle of what would have been the amyloid-beta fragment, producing a harmless piece called p3. In a healthy brain, this non-toxic pathway handles much of the APP processing. In Alzheimer’s, the balance tips toward the beta-secretase pathway, generating excess amyloid-beta that the brain can’t clear fast enough.
From Single Molecules to Plaques
Individual amyloid-beta molecules aren’t inherently dangerous. At very low concentrations (in the picomolar range), they actually participate in normal synaptic function. The problem is accumulation. When amyloid-beta levels rise, the molecules begin sticking to each other. They first form small clusters called oligomers, then longer chains called fibrils, and eventually dense, insoluble plaques visible under a microscope.
The fibril structure is remarkably organized. Sections of the protein chain adopt a flat, sheet-like shape and stack against identical sheets from neighboring molecules through hydrogen bonds, forming what scientists call parallel beta-sheets. A bend in the middle of the chain brings two of these sheets face to face, with their water-repelling surfaces locked together. The result is a tough, rope-like fiber that resists the brain’s normal cleanup systems.
How Plaques Damage Brain Cells
For years, the visible plaques themselves were thought to be the main source of damage. Current evidence points to the smaller, soluble oligomers as the more immediately toxic form. These oligomers interfere with brain cell communication at multiple levels.
At the synapse, where one neuron sends a chemical signal to the next, amyloid-beta oligomers disrupt the machinery that releases neurotransmitters. Specifically, they break apart a protein complex (called SNARE) that drives the release of signaling molecules into the gap between neurons. They also interfere with the protein that recruits essential components to the release site. The net effect is dramatic: direct injection of amyloid-beta oligomers into nerve terminals blocks signal transmission, and even brief exposure to these oligomers can cut the frequency of incoming signals by roughly 50%.
On the receiving side of the synapse, amyloid-beta disrupts receptors for glutamate, the brain’s primary excitatory neurotransmitter. It also affects receptors for acetylcholine, a chemical messenger closely tied to memory and learning. At low concentrations, amyloid-beta can actually enhance acetylcholine receptor activity. At the higher concentrations seen in Alzheimer’s, it shortens the duration of receptor activation, destabilizing the electrical signaling that underlies thought and memory.
Perhaps most importantly, high concentrations of amyloid-beta cause calcium to flood into neurons. Calcium is a critical internal signal, but too much of it damages the cell’s structure and triggers pathways that lead to cell death.
The Connection to Tau Tangles
Alzheimer’s disease involves two signature abnormalities: amyloid plaques outside cells and tau tangles inside them. The dominant theory for the past 25 years, known as the amyloid cascade hypothesis, holds that amyloid-beta buildup is the initial trigger. According to this model, excess amyloid-beta causes tau protein, which normally stabilizes the internal scaffolding of neurons, to become chemically modified in ways that make it detach and clump into tangles. Recent laboratory work supports a direct mechanism: amyloid-beta alters the structural supports inside neurons (microtubules), which in turn promotes tau to become hyperphosphorylated, lose its grip, and aggregate. This cascade leads to the loss of dendritic spines, the small protrusions on neurons where synaptic connections form.
That said, the hypothesis remains debated. Some researchers argue tau pathology can develop independently, and the relationship between the two proteins may be more complex than a simple one-way cascade.
How Amyloid Plaques Are Detected
For most of Alzheimer’s research history, amyloid plaques could only be confirmed after death through brain autopsy. That changed with the development of PET imaging tracers that bind to amyloid deposits in living patients. Three radioactive tracers are currently approved in the U.S. for this purpose: florbetapir (sold as Amyvid), florbetaben (Neuraceq), and flutemetamol (Vizamyl). Each is injected into a vein before a PET scan, and the tracer lights up areas where amyloid has accumulated.
PET scans are effective but expensive, require specialized equipment, and expose patients to a small amount of radiation. A simpler option arrived in 2025 when the FDA cleared the first blood test for detecting amyloid plaques. The test measures the ratio of two proteins in a blood sample: a modified form of tau (pTau217) and a fragment of amyloid-beta. That ratio correlates with whether plaques are present in the brain. In a clinical study of 499 cognitively impaired adults, 91.7% of people who tested positive on the blood test were confirmed to have amyloid plaques by PET scan or spinal fluid analysis. Among those who tested negative, 97.3% were confirmed negative by those same methods. The test is approved for adults 55 and older who are already showing cognitive symptoms, and results are meant to be interpreted alongside other clinical information rather than used as a standalone diagnosis.
Treatments That Target Amyloid
The first generation of Alzheimer’s drugs targeted symptoms, boosting acetylcholine levels or regulating glutamate activity without touching the underlying plaque buildup. A newer class of drugs takes a fundamentally different approach: monoclonal antibodies designed to bind amyloid-beta and help the immune system clear it from the brain.
These antibodies have demonstrated they can substantially reduce plaque levels. In the TRAILBLAZER-ALZ 2 trial, one such drug (donanemab) achieved amyloid clearance in 100% of participants in the lowest plaque groups by 76 weeks of treatment. Plaque burden is measured in centiloids, a standardized scale where zero represents no amyloid and higher numbers indicate greater accumulation. Clearance was defined as dropping below 24.1 centiloids.
Whether clearing plaques translates into meaningful cognitive improvement remains a more nuanced question. These drugs have shown modest slowing of cognitive decline in early-stage Alzheimer’s, on the order of 25 to 35% slower progression compared to placebo over 18 months. They also carry risks, including brain swelling and small brain bleeds that require monitoring with regular MRI scans. For now, they are approved only for people in the early stages of the disease who have confirmed amyloid buildup, and the clinical benefit, while statistically real, is subtle enough that patients and families may not always notice a clear difference in daily life.