Our immune systems work to protect us from illness by recognizing foreign molecules, like those on bacteria or viruses, while not reacting to our own molecules. Bacteria and viruses that cause infection are known as pathogens. When your cells are infected by a pathogen, the pathogen will start to reproduce by making copies of its DNA or RNA (cellular instructions) and will also produce a lot of molecules that fight defense mechanisms of your immune system. The pathogen's DNA and the molecules it produces are foreign to our human cells and, therefore, act as a "danger" signal indicating that something is wrong with the infected cell. This danger signal causes phosphatidylserine to be exposed on the surface of the infected cell to attract macrophages. After macrophages arrive at the infected cell and phagocytose it, they present pieces of the pathogen, known as antigens, to other immune cells, so that the immune system can create a memory of that specific pathogen. If the infected cells are not phagocytosed by macrophages, then more cells, or even the whole organ, can become infected with the pathogen.

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