The wall of an artery is composed of several layers. The lining or inner layer (endothelium) is usually smooth and unbroken. Atherosclerosis begins when the lining is injured or diseased. Then certain white blood cells called monocytes and T cells are activated and move out of the bloodstream and through the lining of an artery into the artery’s wall. Inside the lining, they are transformed into foam cells, which are cells that collect fatty materials, mainly cholesterol. In time, smooth muscle cells move from the middle layer into the lining of the artery’s wall and multiply there. Connective and elastic tissue materials also accumulate there, as may cell debris, cholesterol crystals, and calcium. This accumulation of fat-laden cells, smooth muscle cells, and other materials forms a patchy deposit called an atheroma or atherosclerotic plaque. As they grow, some plaques thicken the artery’s wall and bulge into the channel of the artery. These plaques may narrow or block an artery, reducing or stopping blood flow. Other plaques do not block the artery very much but may split open, triggering a blood clot that suddenly blocks the artery.