We have MHC class one, MHC class two and MHC class three. And in this lecture, we're only going to focus on MHC class one and MHC class two. So let's begin with MHC class one. And this is the diagram that describes this complex, this protein complex found on the cell membrane of a healthy cell. So we essentially have four different polypeptide units that are connected as shown. And we have alpha one, alpha two, alpha three, beta two subunits.