Cholesterol is a lipidic molecule, insoluble in aqueous medium as plasma, in which it circulates in form of pseudo-emulsion: association of lipids and proteins constituting lipoproteins. These lipoproteins vary quantitatively and qualitatively in their lipidic and proteic composition, inducing structural but such a functional difference to them. The most used classification is that which is based on their difference in density. This explains the name of High Density Lipoprotein (HDL), Low Density Lipoprotein (LDL), Very Low Density Lipoprotein (VLDL) and the existence of many intermediate fractions which correspond to all the stages of the lipidic metabolism.
HDL (lipoproteins of high density) contain approximately 50 % of lipids including 20 % of cholesterol. The HDL molecule plays an integral role in removing cellular cholesterol and thus cellular purification. Many epidemiologic studies confirmed the antiatherogen fonction of this fraction leading to the concept of “good cholesterol”. Cholesterol HDL represents consequently an element of evaluation of the atherogenesis risk when there is an imbalance of the ratios cholesterol total/cholesterol HDL or cholesterol LDL/cholesterol HDL.