If we're talking about humans, then the major histocompatibility complex is also known as the human leukocyte antigens. And we'll see why it's given this name in just a moment. Now, there are three different types, three different classes of major histocompatibility complex. 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.