How are artherosclerotic plaques formed?
When something injures the inside of an artery it becomes more permeable to LDL particles ( remember LDL has loads of cholesterol ), which migrate into the arterial wall.
The LDL oxidises and becomes oxidized LDL which is harmful to the body, so, immune system cells go to the site of injury / oxidized LDL. WBCs at the site become macrophages and "consume" the LDL. As they consume more and more LDL they eventually burst, laying out a nice, thick layer of cholesterol / fat. Smooth muscle begins to form over the fat streak, blood cells begin to clot around the area which causes the artery to narrow, decreasing blood flow. This pocket of WBCs, cholesterol / fat, it's smooth muscle covering can burst, sending a blood clot through the artery which can block blood flow to the heart or brain... which is bad.