When a pathogen enters the body, cells in the blood and lymph detect the specific pathogen-associated molecular patterns (PAMPs) on the pathogen's surface. The immune system can be activated by a lot of different things that the body doesn't recognize as its own. These are called antigens. Examples of antigens include the proteins on the surfaces of bacteria, fungi and viruses. ... Sometimes the immune system mistakenly thinks that the body's own cells are foreign cells. Antigens are proteins that are found on the surface of the pathogen. ... The antibodies destroy the antigen (pathogen) which is then engulfed and digested by macrophages. White blood cells can also produce chemicals called antitoxins which destroy the toxins (poisons) some bacteria produce when they have invaded the body.
This process would be the one truly responsible for the "evolution" of mitochondria and chloroplasts as we know them from their prokaryotic ancestors. The cell process that would have initiated endosymbiosis is endocytosis; the process of taking things inside the cell
