Phagocytosis is the process of a cell ingesting another cell or particle. It works by having the phagocytosing cell bind the desired target onto its surface and bringing it inward while it engulfs the target. This is a relatively simple mechanism that cells such as macrophages of the immune system can use to destroy foreign cells and particles. But how do macrophages know what to eat versus what not to eat? Cells can give macrophages a “don’t eat me!” signal so that macrophages will recognize that a cell is not to be eaten and release it.
In addition to macrophages, neutrophils are also able to effectively phagocytose foreign cells and particles. When an area of the body is infected or inflamed, both macrophages and neutrophils are called to that area, partially due to their effectiveness at phagocytosing, which helps in beginning to clear an infection. These “first responders” are vitally important for an effective immune response.
An essential element to phagocytosis is the binding of the target cell or particle with the phagocyte (the phagocytosing cell). This binding can occur via several different mechanisms. One possibility is having the immune system “tag” the target particle or cell with an opsonin as a way of alerting phagocytes that this particle or cell needs to be destroyed. If a cell or particle is coated with an opsonin, opsonin receptors from phagocytes can bind to the opsonins and begin the process of phagocytosis. Another possibility for phagocytosis is the use of toll-like receptors (TLRs). TLRs recognize conserved structures from microbial molecules, and once bound to these structures, triggers an immune response (one possible response, of course, is phagocytosis). There are many different TLRs, each of which has a unique capacity for binding of microbial molecules.
Once a phagocyte has come in contact with a cell or particle and has bound to it, the phagocyte begins to “swallow” the particle or cell. Rather than moving a target through the cell membrane, the cell membrane begins surrounding the target and pulling it inward, eventually completely enclosing the target. At this point, the target is completely inside of the phagocyte, and a phagosome is formed. Next, a phagolysosome is formed by the fusion of the phagosome with lysosome. The pH of this newly formed phagolysosome is dropped so that the target may be killed or neutralized. Upon killing/neutralization, the phagocyte then releases the what’s left of the target back into the body.