Phagocytosis is a process used by cells to engulf and subsequently ingest particles of nutrients or bacteria. This process is a very important part of cell function, allowing cells to grab vital nutrients and allowing the body to protect itself from harmful bacteria. A cell that specializes in this process is known as a phagocyte. It is one among a family of processes collectively referred to under the blanket term “endocytosis,” which refers to any sort of ingestion of material by a cell. The opposite is exocytosis, the expulsion of unwanted material from a cell.
In this process, a cell deforms its membrane to form a little cone around the piece of material that is to be absorbed, and then it closes the sides of the cone, hugging the particle in the cell membrane to create what is known as a phagosome or food vacuole, like a little envelope of material surrounded by the cell membrane. The phagosome, in turn, is passed into the cell for absorption by the lysosomes, the cell structures that specialize in digesting materials that enter the cell. The lysosomes break the phagosome down into its component materials, passing useful compounds on to other structures in the cell and expelling the rest as waste material. In the case of some infectious or harmful material, the phagosome may enter a peroxisome, a special cell structure that helps to rid the body of toxins.
In unicellular organisms, phagocytosis is a critical function, as, without it, the organism will not survive. Some of these organisms have adapted special traits that allow them to track food, orienting themselves in the direction of useful particles that they can ingest. In multicellular organisms, it tends to be a more passive process, but it is still crucial for the survival of the individual cells, ensuring that they get the nutrients they need to function.
Immune system cells also perform phagocytosis, trapping harmful materials when they enter the body and destroying them so that they cannot cause damage. In some cases, the body may respond with inflammation, as a flood of immune system cells rushes to a location to deal with unwanted invaders. When the immune system can no longer cope with harmful materials, or when these materials resist the ingestion process, the consequences for the host body can be quite unpleasant.