Let's suppose that we have the common flu and what that means is a special type of viral agent known as influenza infected our body. Now, what this virus does is it infects our healthy cells and it creates infected cells. So the infected cell begins to produce the viral agents that are needed by the virus to essentially reproduce. But what the infected cell also does is it takes a piece of that virus, some type of antigen and places it onto the major histocompatibility complex, class one as shown in the diagram. So this is the MHC class one of the infected cell. And this green portion is the antigen that came from the viral agent that infected our cell.