# Chapter Three: Cell biology

### Cell membrane - Function - Marker Proteins

| Marker proteins extend across the cell membrane and serve to identify the cell. The immune system uses these proteins to tell friendly cells from foreign invaders. They are as unique as fingerprints. They play an important role in organ transplants. If the marker proteins on a transplanted organ are differnt from those of the original organ the body will reject it as a foreign invader.

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