Within the body there are three levels of defense which can be divided in to specific, non-specific, non-cell mediated and cell cell mediated. Depending on the level of immunity, will also depend which types of defenses will be used.
Self cells are so called because they are endemic to the body and are distinguished by Major histocompatibility complex (MHC) Class I structures on the external surface of the cell. Non-self or foreign cells are recognised because of their lack of the necessary MHC markers.