LDL bound to receptor. Computer illustration of a low-density lipoprotein (LDL), or 'bad' cholesterol, molecule (round) bound to an LDL receptor (LDLR) protein (Y-shaped), with red blood cells (erythrocytes, red) in a blood vessel (not to scale). Each molecule of LDL consists of a core of esterified cholesterol molecules surrounded by a shell (green and yellow) of unesterified cholesterol and phospholipids. The complex structure includes carrier proteins (blue) known as apo-B proteins, which assist transport in the blood. When there are high levels of LDL cholesterol in the blood it builds up on the sides of blood vessels hardening them, a condition named atherosclerosis. This narrows the blood vessels and may block them. LDL receptors recognise and bind to LDL molecules to remove them from the bloodstream.