Cholesterol is an important lipid, which in spite of its bad press, is an essential nutrient necessary for repairing cell membranes, manufacturing vitamin D on the skin's surface, and creating hormones, such as estrogen and testosterone. The body acquires some cholesterol through diet, but about two-thirds is manufactured in the liver, its production stimulated by saturated fat. Cholesterol and some other lipids, including triglycerides, are transported in the blood packed in sphere-shaped bodies called lipoproteins. LDL does, however, penetrate the walls of the artery where it can interact dangerously through a process called oxidation, caused by unstable molecules called oxygen-free radicals. These particles are released naturally during the body's chemical processes and increase when the body is exposed to environmental toxins, such as cigarette smoke.
Free radicals are essential in fighting bacteria, but in excess they can cause harm in many ways.
Because they are missing an electron, free radicals tend to bind with any other molecule in the body and can thus become destructive. If LDL collects on arterial walls, free radicals released from the wall membranes attack and modify its form. The resulting oxidized form of LDL triggers white blood cells in the immune system to gather at the site, forming a fatty substance called plaque and causing inflammation. As the plaque builds up, the arterial walls slowly constrict, reducing blood flow. In addition, the body forms calcium to wall off the inflamed area in the artery.