LDL cholesterol, referred to as "bad" cholesterol, deposits in the walls of arteries, increasing a person's risk for heart attack or stroke, states the American Heart Association. HDL cholesterol, or "good" cholesterol, works to take LDL cholesterol out of the arteries and back to the liver to be broken down.
LDL also contributes to plaque formation inside the arteries when white blood cells try to digest the LDL cholesterol in the bloodstream, explains WebMD. The white blood cells convert LDL into a toxic form, leading to inflammation of the artery. More LDL cholesterol and other cells eventually collect around the inflamed area and form a plaque, or bump, in the artery wall. This plaque can slowly grow large enough to block the artery, or it may suddenly rupture, potentially leading to a heart attack. Overall, higher levels of LDL in the blood increase a person's risk of developing heart disease.
Because HDL removes excess cholesterol from the blood, higher HDL levels usually lead to lower LDL levels, states Mayo Clinic. Maintaining a healthy amount of HDL cholesterol in the blood is associated with a lower risk of heart disease and stroke, whereas low levels of HDL may increase the risk of heart disease, states the American Heart Association.