Little attention has been paid to the function of lipoproteins as part of a nonspecific immune defense system that binds and inactivates microbes and their toxins effectively by complex formation. Because of high extra-capillary tissue pressure, aggregates of such complexes may be trapped in vasa vasorum of the major arteries. This complex formation and aggregation may be enhanced by hyperhomocysteinemia, because homocysteine thiolactone reacts with the free amino groups of apo-B to form homocysteinylated low-density lipoprotein (LDL), which is subject to spontaneous precipitation in vitro. Obstruction of the circulation in vasa vasorum, caused by the aggregated complexes, may result in local ischemia in the arterial wall, intramural cell death, bursting of the capillary, and escape of microorganisms into the intima, all of which lead to inflammation and creation of vulnerable plaques. The presence of homocysteinylated LDL and oxidized LDL stimulates production of LDL autoantibodies, which may start a vicious circle by increasing the complex formation and aggregation of lipoproteins. The content of necrotic debris and leukocytes and the higher temperature than its surroundings give the vulnerable plaque some characteristics of a micro-abscess that by rupturing may initiate an occluding thrombosis. This suggested chain of events explains why many of the clinical symptoms and laboratory findings in acute myocardial infarction are similar to those seen in infectious diseases. It explains the presence of microorganisms in atherosclerotic plaques and why bacteriemia and sepsis are often seen in myocardial infarction complicated with cardiogenic shock. It explains the many associations between infections and cardiovascular disease. And it explains why cholesterol accumulates in the arterial wall. Some risk factors may not cause vascular disease directly, but they may impair the immune system, promote microbial growth, or cause hyperhomocysteinemia, leading to vulnerable plaques.