How do cells recognise each other?
Each of your cells has a set of 'identity tags' on its surface, which mark it out as part of your body and no one else's. Some are only found on cells from the same tissue or organ. These identity tag molecules are called antigens. Your set of antigens is unique, unless you have an identical twin. Your immune system recognises invading germs because they have unfamiliar antigens on their surfaces.