This is a pathogen, for example, a bacterial cell. Notice that both of these cells actually contain these membrane bound macromolecules. For example, let's suppose they're lipoproteins. Now, these lipoproteins that the human cell contains are easily recognized by the cells of our immune system. And so these membrane protein of human cells can be used by the immune system in self recognition and that ensures that our own immune system doesn't actually destroy our own cells. On the other hand, these foreign macromolecules, or antigens, can be recognized by the immune cells as foreign objects.