Atherosclerosis starts when LDL cholesterol slips from the bloodstream into the inner lining of an artery through tiny gaps created by ongoing stress, such as high blood pressure or inflammation. Once lodged beneath the surface, LDL becomes chemically altered. The immune system treats this altered cholesterol as a threat and mounts a local inflammatory response. Monocytes move into the vessel wall and transform into macrophages that engulf the altered LDL. As they fill with fat, these cells beco