Lipoproteins. Illustration of a low density lipoprotein (LDL), or 'bad' cholesterol, molecule (left) and a high density lipoprotein (HDL), or 'good' cholesterol, molecule (right). Lipoproteins consists of a core of esterified cholesterol molecules surrounded by a shell (blue) of unesterified cholesterol and phospholipids. The complex structure includes carrier proteins (pink) known as apolipoproteins, which assist transport in the blood. HDL cholesterol plays a role in fat metabolism and contributes to cardiovascular health. 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.