The cellular response to infection – phagocytosis.

Phagocytosis is the ingestion and destruction of microbes by cells called phagocytes. The two main types of phagocytes are the macrophages and neutrophils described above. The way in which macrophages and neutrophils phagocytose particles is essentially the same and can be divided into four stages.

1. Attachment of the phagocyte to the particle being phagocytosed, which may be a pathogen, a dead or damaged host cell or a piece of tissue.
2. Ingestion. By extending membrane protrusions called pseudopodia around the particle, the phagocyte is able to engulf the particle, which is taken into the cell in a phagocytic vacuole.
3. Killing. If the ingested particle is a live cell of a pathogen (e.g. a bacterium) the phagocyte will normally kill the cell by one of a number of mechanisms.
4. Degradation. The phagocytosed particle, whether it is a dead cell or a piece of tissue, is broken down by enzymes in the phagocytic vacuole.

 Phagocytosis. Phagocytes can take up and remove bacteria and dead host cells or tissue debris. The figure shows phagocytosis of a bacterium: ➀ The phagocyte binds to the bacterium. ➁ The phagocyte extends projections around the bacterium and engulfs it in a phagocytic vacuole. ➂ The phagocyte kills the engulfed bacterium. ➃ The bacterium is degraded by proteolytic enzymes.

Although the basic process of phagocytosis is similar in neutrophils and macrophages, there is an important difference. While neutrophils are only able to phagocytose small organisms such as bacteria and viruses, macrophages are able to phagocytose larger particles such as dead cells and tissue debris in addition to microorganisms. Therefore macrophages are involved in eliminating pathogens from tissues and also in cleaning up damaged tissue by removing dead or damaged host cells. Macrophages are able to distinguish between healthy host cells and dead/damaged cells because the receptors they have for recognising sugars on microbes also recognise sugars that are exposed by dead or damaged host cells.