A marker protein exists in the cell membrane and serves as the identifier of the cell. The protein analyzes any invaders, binding itself onto them and inducing a visual response to make its decision.
A body's immune system uses marker proteins to distinguish friendly cells from foreign invaders. Marker proteins are all unique identifiers, so much like human fingerprints, no two are the same.
These proteins play an important role in things like human organ transplant. If the marker proteins on the transplanted organ are different from those from the original organ, the body will reject it as a foreign invader.