The specific residue onto which GalNAc will be attached is not defined, because there are numerous enzymes that can add the sugar and each one will favour different residues. However, there are often proline (Pro) residues near the threonine or serine. Once this initial sugar has been added, other glycosyltransferases can catalyse the addition of additional sugars. Two of the most common structures formed are Core 1 and Core 2. Core 1 is formed by the addition of a galactose sugar onto the initial GalNAc. Core 2 consists of a Core 1 structure with an additional "N"-acetylglucosamine (GlcNAc) sugar. A poly-"N"-acetyllactosamine structure can be formed by the alternating addition of GlcNAc and galactose sugars onto the GalNAc sugar. Terminal sugars on O-glycans are important in recognition by lectins and play a key role in the immune system. Addition of fucose sugars by fucosyltransferases forms Lewis epitopes and the scaffold for blood group determinants. Addition of a fucose alone creates the H-antigen, present in people with blood type O. By adding a galactose onto this structure, the B-antigen of blood group B is created. Alternatively, adding a GalNAc sugar will create the A-antigen for blood group A.  "O"-GalNAc sugars are important in a variety of processes, including leukocyte circulation during an immune response, fertilisation, and protection against invading microbes. "O"-GalNAc sugars are common on membrane glycoproteins, where they help increase rigidity of the region close to the membrane so that the protein extends away from the surface. For example, the low-density lipoprotein receptor (LDL) is projected from the cell surface by a region rigidified by O-glycans. In order for leukocytes of the immune system to move into infected cells, they have to interact with these cells through receptors. Leukocytes express ligands on their cell surface to allow this interaction to occur. P-selectin glycoprotein ligand-1 (PSGL-1) is such a ligand, and contains a lot of O-glycans that are necessary for its function. O-glycans near the membrane maintain the elongated structure and a terminal sLex epitope is necessary for interactions with the receptor. Mucins are a group of heavily O-glycosylated proteins that line the gastrointestinal and respiratory tracts to protect these regions from infection.