Cholesterol particles. Computer artwork of low- density (LDL, right) and high-density (HDL, left) lipoproteins. These particles are the major carriers of cholesterol in the blood. Cholesterol (purple spheres) is carried as an ester inside the particles, surrounded by a layer of phospholipids (yellow) and unesterified cholesterols. A transmembrane protein (purple and pink strands) organizes the particle and is recognised by target cells. LDL cholesterol is often called 'bad' cholesterol because high levels in the blood lead to increased risk of atherosclerosis, heart disease and stroke.