Well, it's a complex of proteins found on the membrane of our cells that allows this process to take place. And the complex of proteins is known as the major histocompatibility complex or simply MHC. Now, specifically in humans because MHC is found in all animals. 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.