Transferrin is a serum protein that carries iron through the bloodstream to the bone marrow, where red blood cells are manufactured, as well as to the liver and spleen. Structurally, transferrin is a polypeptide with two N-linked polysaccharide chains. These polysaccharide chains are branched with sialic acid residues.  Sialic acid is a monosaccharide carbohydrate.
Various forms of transferrin exist, with differing levels of sialylation.  The most common form is tetrasialotransferrin, with four sialic acid chains.  In persons who consume significant quantities of alcohol (usually more than 4 or 5 alcoholic beverages a day for two weeks or more) , the proportion of transferrin with zero, one, or two sialic acid chains is increased.  These are referred to as carbohydrate-deficient transferrins.  These carbohydrate-deficient transferrins can be measured in the bloodstream, and are important markers for alcohol use disorder.