LDL and HDL particles, illustration - stock illustration

Lipoproteins. Illustration of a low density lipoprotein (LDL), or bad cholesterol, molecule (left) and a high density lipoprotein (HDL), or good cholesterol, molecule (right), showing their comparative sizes. Lipoproteins consists of a core of esterified cholesterol molecules surrounded by a shell of unesterified cholesterol (orange with violet cap) and phospholipids (orange with blue cap). The complex structure includes carrier proteins (blue and green) 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.