How is the body able to recognize that transplanted organs are foreign?

The body recognizes transplanted organs as foreign primarily through the immune system, which identifies differences in the organ's antigens—molecules on the surface of cells that signal self or non-self. T cells and B cells are activated by these foreign antigens, leading to an immune response that can attack and reject the transplanted organ. This recognition is a crucial aspect of the body’s defense mechanism against pathogens, but it also poses a challenge in organ transplantation, necessitating immunosuppressive therapies to prevent rejection.