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Low density lipoproteins (LDL) are sometimes called the "bad cholesterol". LDL levels are strongly correlated with increased cardiovascular risk. At high concentrations, LDLs accumulate on the outer surface of the blood vessels and are subsequently oxidized. In the oxidized form, the LDL can enter the tunica intima, the linings of the blood vessels. The endothelial cells of the blood vessels secrete MCP-1, a chemoattractant protein for monocytes. Monocytes infiltrate the tunica intima and become macrophages. Ingestion of the concentrated oxidized LDL transforms the macrophages to foam cells. A fatty streak along the vessels is characteristic of the foam cells.
The lesion of the endothelial lining, accumulation of foam cells, and the concomitant growth of smooth muscle cells lead to the thickening of the blood vessels. The smooth muscle cells also absorb the oxidized LDL and become foam cells themselves. The process continues, eventually leading to a blocked vessel.
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