Recall that our immune system attacks anything that it recognizes as foreign or pathogenic. And if the major histocompatibility complex MHC self antigens of the transplanted tissue cells do not match or not compatible with the host cells, then the host immune system will mount a defensive response and destroy that allograph. And we see that take place in the following diagram. 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.