Body components, including lipids, are continually being formed, used, degraded and replaced. The process entails transport of biological compounds between tissues, using the blood plasma as the main highway.
For lipids, this poses a problem since they’re generally not water-soluble.
The transport problem has been solved by incorporating cholesterol and triglycerides into the lipoproteins. (See "Other lipids - phospholipids".) These lipoproteins are classified according to their density or protein content.
LDL and HDL are the two varieties of lipoproteins that are most strongly associated with vascular disease.
HDL (“good” cholesterol)
An increased level of HDL cholesterol is associated with a lower risk of coronary heart disease (CHD). Conversely, low levels of HDL cholesterol are associated with an increased risk. This is why HDL is thought of as the "good" type of cholesterol.
However, it seems that it is not actually the cholesterol itself that is “good”, but the lipoprotein which carries it – in other words, high-density lipoprotein (HDL).
It appears that the HDL molecule itself can clean out excess cholesterol – including that which came from the “bad” LDL cholesterol which has accumulated in the walls of arteries – and take it back to the liver for reprocessing. It also performs antioxidant activities, which help protect arteries against atherosclerosis, the cause of CHD.