High-density lipoprotein (HDL) is a “mixed particle” consisting of lipids and proteins, often referred to as “good cholesterol”. The following is an introduction to HDL from two aspects: structure and function and factors affecting concentration: 1. Structure and function Cholesterol is one of the important blood lipids. HDL is a type of lipoprotein whose main function is to remove cholesterol from atherosclerotic plaques or other body tissues and transport it to the liver for metabolism and excretion from the body. Increased levels of HDL reduce the risk of myocardial infarction, while decreased levels increase the risk of atherosclerosis. Therefore, HDL is often referred to as “good cholesterol”. 2. Influencing factors HDL level is obviously influenced by genetic factors; severe malnutrition is accompanied by a significant decrease in total serum cholesterol and low HDL. Obese people have low HDL due to possible metabolic disorders. Patients with hypertriglyceridemia are often associated with lower HDL due to disease states such as smoking, diabetes, hepatitis and cirrhosis. In contrast, exercise and small amounts of alcohol will raise HDL.