So let's suppose this is our host cell. And the host cell will contain special major histocompatibility complexes on the membrane, as shown in blue. And it will take some type of protein, some type of self antigen and place it onto that complex, as shown in green. And the host immune cells, for example, cytotoxic T cells will be able to distinguish this host cell from some type of foreign pathogen as a result of these self antigens present on the MHC protein. Now, if we basically transplant a graft, a tissue or an organ that contains cells with a different MHC protein, with a different major histocompatibility complex that contains a different cell antigen, then the host immune cell will label this cell as being pathogenic, as being foreign. And what will happen is this cell will destroy that graft cell along with that entire allograft.