Lipoproteins are complexes of lipids and proteins that transport cholesterol, triglycerides, and fat-soluble vitamins throughout the body. They can be classified based on density and size into different classes, including chylomicrons, VLDLs, IDLs, LDLs, and HDLs. The major apolipoproteins associated with each lipoprotein class help to assemble particles and activate enzymes involved in lipoprotein metabolism. Disorders can cause abnormal accumulation of certain lipoprotein classes and are classified according to the Frederickson system.