How does living in phagocytes allow some pathogens to escape the immune system?
Phagocytes are cells that take something into themselves, or “eat” it. The immune system includes several phagocytes that take bacteria and other pathogens into themselves and attempt to digest the pathogen. Most of your white blood cells are some type of phagocyte, including neutrophils and macrophages.
Most pathogens cannot live inside a phagocyte. But those that do are very well hidden from other cells in the immune system, as well as antibodies that are circulating in the body outside the phagocyte.
Usually, a phagocyte works by encircling a pathogen within a membranous sac (called a phagosome) and bringing it into itself. Then, other sacs in the phagocyte that is filled with digestive enzymes (called lysosomes) will fuse with the phagosome and spill their enzymes into it, to destroy the pathogen. Any remaining waste products can then be expelled from the cell.
However, some pathogens avoid this death by escaping the phagosome and living free inside the cytoplasm of the phagocyte. The digestive enzymes in the lysosome sacs can’t find the pathogen to destroy it, because the lysosome can only recognize and fuse with a phagosome sac. (And you wouldn’t want the digestive enzymes to get out of the sacs to reach the pathogen, because then the enzymes would damage the phagocyte’s innards.)
Instead of escaping the phagosome, some other pathogens can prevent lysosomes from fusing with the phagosome. They can then live and grow inside the phagosome unbothered. This also helps them hide from the rest of the immune system, as they are hidden within the white blood cell while the white blood cell can’t kill them.
Some pathogens can’t escape the phagosome and can’t prevent the lysosomes from fusing with it, but they can still survive because they’re resistant to the digestive enzymes. In fact, a few pathogens actually prefer to have those enzymes around, and grow better with them!
Overall, living inside a phagocyte lets a pathogen hide from the rest of the body’s immune system, just so long as the pathogen has some way to survive living within the phagocyte.
