In its native form, LDL is not harmful but it is a process called oxidation that causes LDL to become harmful. The mechanism for the development of atherosclerotic plaques is still not fully resolved. The proposal is as follows. When LDL enters normal arteries, an immune response is triggered. This immune response sends 'signals' to circularing immune cells called monocytes.
Monocytes are the immature form of macrophages.
Macrophages are professional scavengers that assist in 'devoring rubbish'. A good example is in the lungs where minute particles may get trapped in the airways and cause allergic responses. The monocytes are attracted by these allergic particles and they mature rapidly into macrophages in the airways. The macrophages have a very special capability called phagocytosis. Phagocytosis is a method whereby the macrophages can ingest particles or debri much greater than their size.