A skin graft coming from another person would be recognized by the bodys immune system as an attack from a foreign invader. The term antigen comes from the term "antibody generator", which is part of the immune system responsible for generating antibodies that will specifically attack and destroy what the body perceives as invading bacteria, viruses, or other foreign matter. In this case, it happens to be a skin graft. So, rather than accepting the skin and adapting it to the body, the immune system sets about the manufacture of antibodies specific to the receptors on the skin graft. The antibodies attack the skin graft just as they would any other foreign material introduced into the body. The skin graft is ultimately degenerated and discarded as dead tissue. Skin grafts must be from donors that are as close genetically to the recipient as possible to avoid this type of internal conflict from the immune system.