LDL bound to receptors. Computer illustration of low-density lipoprotein (LDL), or 'bad' cholesterol, molecules (round) bound to LDL receptor (LDLR) proteins (Y-shaped) in a cell surface (bottom). Each molecule 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 (thread-like) 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.