Atherosclerosis develops when cholesterol-carrying particles accumulate inside artery walls, triggering a decades-long cycle of inflammation, immune cell buildup, and scar tissue formation that gradually narrows the vessel. The process begins surprisingly early: the Bogalusa Heart Study found fatty streaks in the aortas of every child examined and in the coronary arteries of half the children aged two to fifteen. Yet the disease typically causes no symptoms until middle age or later, when a plaque grows large enough to restrict blood flow or, more dangerously, ruptures and triggers a clot.
It Starts With Damage to the Artery Lining
The inner surface of every artery is lined with a single layer of cells called the endothelium. When these cells are healthy, they act as a selective barrier, controlling what passes from the bloodstream into the artery wall. Atherosclerosis begins when something compromises that barrier.
Several forces can do the damage. High blood pressure, smoking, high blood sugar, and chronic inflammation all injure endothelial cells directly. But the geometry of your arteries matters too. At branch points and curves, blood flow becomes turbulent instead of smooth. This disturbed flow changes how the lining cells behave, pushing them toward a pro-inflammatory state where they produce more adhesion molecules and signaling chemicals that attract immune cells. That’s why plaques tend to cluster at arterial forks and bends rather than forming evenly throughout the body.
Once the lining is compromised, it becomes more permeable. LDL cholesterol particles, which are small enough to slip through, begin accumulating in the artery wall beneath the surface. This subendothelial buildup of LDL is the critical first step. The trapped LDL doesn’t just sit there passively. It undergoes chemical changes, primarily oxidation, that make it far more inflammatory and attractive to immune cells.
Immune Cells Create Foam Cells
The damaged endothelial cells send out distress signals, including inflammatory molecules that recruit white blood cells from the bloodstream. Monocytes, a type of immune cell, respond to these signals by crossing the artery lining and entering the wall. Once inside, they mature into macrophages, which are essentially cleanup cells designed to engulf debris and pathogens.
Here’s where the process goes wrong. Normal LDL is taken up by cells in a carefully regulated way, with feedback mechanisms that stop the cell from absorbing too much cholesterol. But oxidized LDL bypasses those controls. Macrophages engulf it through specialized surface receptors, most notably one called CD36, that have no such off switch. The macrophage keeps swallowing oxidized LDL until it’s engorged with fatty droplets. Under a microscope, these bloated cells look foamy, which is why they’re called foam cells.
Foam cells are the building blocks of early plaque. As they accumulate, they form visible fatty streaks beneath the artery surface. These streaks are already present in children and teenagers, though they cause no harm at this stage. The trouble is that foam cells don’t just store fat quietly. They release inflammatory signals that recruit still more immune cells, creating a self-reinforcing loop of inflammation and lipid accumulation.
Plaque Grows a Protective Cap
As the fatty core of the plaque expands, the body attempts a repair. Smooth muscle cells that normally live in the middle layer of the artery wall receive inflammatory signals, particularly a molecule called IL-6, that prompt them to migrate inward toward the growing lesion. Once there, they multiply and begin producing collagen and other structural proteins that form a tough, fibrous cap over the lipid core.
This cap is a stabilizing structure. A thick, collagen-rich cap keeps the contents of the plaque sealed off from the bloodstream and dramatically reduces the risk of a sudden cardiovascular event. In many people, plaques remain stable for years or even a lifetime, never causing symptoms beyond a gradual narrowing of the artery. The balance between cap building and cap destruction is what ultimately determines whether a plaque stays quiet or becomes dangerous.
What Makes a Plaque Dangerous
Not all plaques are equal. The ones most likely to cause heart attacks or strokes are often not the largest. They’re the ones with thin, fragile caps and large pools of soft, lipid-rich material underneath. A plaque is classified as a thin-cap fibroatheroma when its fibrous covering is less than 65 micrometers thick, roughly the width of a human hair. Studies using imaging inside coronary arteries suggest that caps below about 80 micrometers carry significantly elevated risk of rupture.
Two processes thin the cap simultaneously. First, immune cells inside the plaque, especially T-lymphocytes, release a signaling molecule called interferon-gamma that directly inhibits smooth muscle cells from producing new collagen. The cap stops being replenished. Second, activated immune cells produce a family of enzymes that actively digest existing collagen fibers. These enzymes make the initial cut in the collagen structure, and a second group of enzymes finishes breaking down the fragments. The result is a cap that’s being eaten away from within while its repair crew has been shut down.
When the cap finally tears, blood is exposed to the highly thrombogenic contents of the lipid core. A clot forms rapidly at the rupture site. If that clot is large enough to block the artery, the tissue downstream loses its blood supply. In a coronary artery, this is a heart attack. In an artery supplying the brain, it’s a stroke. Not all ruptures cause complete blockages. Some produce small clots that heal over, adding another layer to the plaque and accelerating its growth in a stepwise fashion.
Rupture isn’t the only mechanism. Some dangerous events occur through erosion, where the endothelial surface over the plaque wears away without the cap fracturing. Enzymes that attack the basement membrane beneath endothelial cells can cause them to detach and die, exposing the underlying tissue to blood and triggering clot formation even without a deep rupture.
Calcification in Advanced Plaques
As plaques age, they often develop calcium deposits through a process that resembles bone formation. Smooth muscle cells within the plaque can shift into a bone-building mode, driven in part by the same signaling pathway that controls bone metabolism throughout the body. A molecule called RANKL binds to receptors on smooth muscle cells and activates a cascade that leads to mineral deposition within the plaque. The body has a natural counterbalance, a decoy receptor called OPG, that can intercept RANKL and block this process.
Calcification is a double-edged feature. Heavy, sheet-like calcification can actually stabilize a plaque by making it rigid. But small, spotty calcium deposits within a soft lipid core may concentrate mechanical stress on the fibrous cap and increase vulnerability. This is why a high coronary calcium score on a CT scan indicates significant plaque burden but doesn’t by itself tell you whether those plaques are stable or fragile.
A Disease That Builds for Decades
The full arc of atherosclerosis typically spans 30 to 50 years. Fatty streaks appear in childhood. Through the teens and twenties, some of these progress into more organized plaques with lipid cores and early fibrous caps, especially in people with risk factors like high cholesterol, smoking, obesity, or diabetes. By the forties and fifties, plaques may be large enough to limit blood flow during exertion, causing symptoms like chest pain with exercise. The most serious complications, heart attacks and strokes, peak from the fifties onward, though they can strike earlier in people with aggressive risk factor profiles.
Because the disease is silent for so long, markers of underlying inflammation can help gauge risk before symptoms appear. High-sensitivity C-reactive protein, a blood test that reflects systemic inflammation, is one such marker. Levels below 1 mg/L are considered low risk, 1 to 3 mg/L intermediate, and above 3 mg/L high risk for cardiovascular events.
How LDL Targets Shape Treatment
Since LDL accumulation in the artery wall is the initiating event, lowering LDL cholesterol is the cornerstone of slowing or partially reversing atherosclerosis. The most recent guidelines from the American College of Cardiology and the American Heart Association set aggressive targets for people who already have cardiovascular disease. The goal for most of these patients is an LDL below 55 mg/dL, with at least a 50% reduction from their starting level. For those at somewhat lower risk, the initial target is below 70 mg/dL, though the guidelines note that the majority of people with a history of cardiovascular events will qualify for the stricter 55 mg/dL goal.
These targets reflect a simple biological reality: the less LDL circulating in your blood, the less gets trapped in artery walls, and the slower existing plaques grow. At very low LDL levels, some plaques even shrink modestly as lipid is gradually removed from the core, thickening the fibrous cap and making rupture less likely. The process of atherosclerosis is relentless but not irreversible, and every stage of its development offers a point where reducing risk factors can change the trajectory.