What is the function of identifier proteins?

To indentify a cell. For exmple, if you received blood from a donor because you lost some, the cells that are transplanted into the body, must identify themselves to the body. If the ID checks out, they are free to do what they must. However, if it does not check out (which is usually the case in blood donations), the immune system attacks the invading cells and destroys them. Identifier proteins are almost as unique to an individual as their fingerprint.