Lipoprotein in the intestine, illustration. This is low density lipoprotein (LDL), or 'bad' cholesterol. 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 (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.