Apolipoproteins are proteins that bind to lipids (fats) to form lipoproteins, which transport fats through the bloodstream. They play a crucial role in lipid metabolism and help regulate cholesterol levels in the body. Different types of apolipoproteins, such as ApoA-I and ApoB, have specific functions and are associated with various lipoproteins like HDL (high-density lipoprotein) and LDL (low-density lipoprotein). These proteins also serve as ligands for receptors on cell surfaces, facilitating the uptake of lipoproteins by cells. By influencing lipid transport and metabolism, apolipoproteins are important for maintaining cardiovascular health and preventing conditions like atherosclerosis.