Phagocytosis is the transport mechanism that can bring whole cells into a cell. It is a form of endocytosis specifically designed to engulf large particles, anything larger than 0.5 micrometers in diameter, including entire bacteria, dead cells, and even other living cells. While other transport methods like diffusion or channel-mediated transport move individual molecules, phagocytosis is the only standard mechanism capable of internalizing something as large as another cell.
How Phagocytosis Works
The process begins when a cell detects a target on its surface through specialized receptors. Once the target is recognized, the cell’s internal scaffolding (a protein network called actin) rapidly reorganizes to push the cell membrane outward, forming arm-like extensions called pseudopods. These extensions wrap around the target like a pair of arms closing around a ball, creating a cup-shaped structure that progressively tightens around the particle.
The pseudopods continue extending until they meet on the far side of the target, fully enclosing it. At that point, the membrane pinches off inward, pulling the engulfed cell into the interior inside a membrane-bound compartment called a phagosome. This is now a bubble floating inside the cell, completely sealed off from the outside environment. The tightening and elongation of the cup depends on motor proteins that grip the target and pull the membrane snugly around it. When those motor proteins are blocked in experiments, the cup still forms but stays loose and floppy, unable to close.
What Happens After Engulfment
The phagosome doesn’t just sit there. It migrates toward the center of the cell, where it progressively fuses with other internal compartments called lysosomes. Each fusion event makes the interior of the phagosome more acidic and fills it with digestive enzymes. The final product, called a phagolysosome, is an intensely acidic, enzyme-rich environment with full capacity to break down and kill whatever was swallowed.
This maturation process happens in stages. The phagosome first interacts with early sorting compartments, then with later ones, each time becoming a harsher environment. By the time it merges fully with lysosomes, the engulfed cell or microbe is efficiently destroyed and its components recycled.
Which Cells Perform Phagocytosis
Technically, almost all cells in the body have some ability to perform phagocytosis. But only a handful do it well enough to matter. These “professional phagocytes” include macrophages, neutrophils, monocytes, dendritic cells, and osteoclasts. Macrophages and neutrophils are the heavy hitters of the immune system, constantly patrolling tissues and blood for bacteria, dead cells, and debris.
Macrophages can ingest surprisingly large targets. Laboratory experiments show they can engulf particles up to 15 micrometers in diameter, though targets that large require the surface to be coated with antibodies first, and the process takes about 30 minutes to complete. For comparison, most bacteria are 1 to 5 micrometers, well within the comfortable range.
Pathogens That Exploit Phagocytosis
Some bacteria have evolved to use phagocytosis as a front door. Salmonella, for example, deliberately gets ingested by immune cells in the gut. Once inside, instead of being destroyed, it survives within the phagosome and hitches a ride as the immune cell travels through the body. More than 80% of Salmonella found in infected tissues are living inside professional phagocytes. Mutant strains of Salmonella that cannot survive inside macrophages also cannot establish infections in animal models, confirming that riding inside immune cells is central to how the bacterium spreads.
Tuberculosis works similarly. Mycobacterium tuberculosis gets engulfed by macrophages in the lungs and then blocks the phagosome from fusing properly with lysosomes, preventing the cell from completing its digestion process. The bacterium essentially locks itself in a safe room inside the very cell that was supposed to kill it.
Entosis: A Non-Phagocytic Alternative
Phagocytosis isn’t the only way a whole cell can end up inside another. A process called entosis allows one living cell to actively invade a neighboring cell, creating a “cell-in-cell” structure. Unlike phagocytosis, where the engulfing cell does the work, entosis is driven by the invading cell itself. The invading cell uses its own internal machinery to push into its neighbor, exhibiting a blebbing, amoeba-like motion.
Entosis relies on completely different molecular equipment than phagocytosis. It requires cell-to-cell adhesion junctions (the kind normally found in skin and organ lining cells) rather than immune receptors. The invading cell is typically alive and healthy at the time of entry, though it is usually killed and digested afterward. Structures resembling entosis have been documented in breast, colon, liver, and pancreatic cancers, suggesting the process plays a role in tumor biology. Still, entosis is far less common and less well understood than phagocytosis.
Phagocytosis and the Origin of Complex Cells
The ability to swallow one cell whole may be responsible for the existence of complex life itself. Endosymbiotic theory, supported by over a century of evidence, holds that mitochondria (the energy-producing structures inside your cells) originated when an ancient cell engulfed a bacterium and, instead of digesting it, kept it alive. The bacterium provided efficient energy production, and over billions of years it became a permanent internal component. The same process happened again later when a cell that already had mitochondria engulfed a photosynthetic cyanobacterium, eventually giving rise to the chloroplasts in plant cells.
More than 20 different versions of this theory have been proposed, but the core idea is the same: cells united, one inside the other, to create entirely new lineages of life. Without a mechanism for bringing whole cells inside other cells, eukaryotic life (everything from yeast to humans) may never have evolved.