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Source: Wikipedia.org article, adapted under https://creativecommons.org/licenses/by-sa/3.0/ license.

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Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein. These groups, from least dense, compared to surrounding water, (largest particles) to most dense (smallest particles), are chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein and high-density lipoprotein (HDL).

Lipoproteins transfer lipids (fats) around the body in the extracellular fluid thereby facilitating fats to be available and taken up by the cells body wide via receptor-mediated endocytosis. Lipoproteins are complex particles composed of multiple proteins, typically 80-100 proteins/particle (organized by a single apolipoprotein B for LDL and the larger particles). A single LDL particle is about 220-275 angstroms in diameter typically transporting 3,000 to 6,000 fat molecules/particle, varying in size according to the number and mix of fat molecules contained within. The lipids carried include all fat molecules with cholesterol, phospholipids, and triglycerides dominant; amounts of each varying considerably. Lipoproteins can be sampled from blood for evaluation of atherosclerosis driving factors.

LDL particles pose a risk for cardiovascular disease when they invade the endothelium and become oxidized, since the oxidized forms are more easily retained by the proteoglycans. A complex set of biochemical reactions regulates the oxidation of LDL particles, chiefly stimulated by presence of necrotic cell debris and free radicals in the endothelium. Increasing concentrations of LDL particles are strongly associated with increasing rates of accumulation of atherosclerosis within the walls of arteries over time, eventually resulting in sudden plaque ruptures, decades later, and triggering clots within the artery opening; these debris & clots narrowing or closing off the opening locally (more commonly microscopic branches distal to plaque rupture locations), i.e. cardiovascular disease, stroke, and other vascular disease complications.

LDL particles (though far different from cholesterol per se) are sometimes referred to as bad cholesterol because they can transport their content of lipid molecules into artery walls, attract macrophages, and thus drive atherosclerosis. In contrast, HDL particles are often called good cholesterol or healthy cholesterol because they can remove lipid molecules from macrophages in the wall of arteries.

A hereditary form of high LDL is familial hypercholesterolemia (FH). High LDL is termed hyperlipoproteinemia type II (after the dated Fredrickson classification).