15-12 Phagocytes engulf and kill foreign cells

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• Phagocytes move about the body, are recruited and activated by inflammation, attach to invading pathogens, engulf them, and kill them.
• Neutrophils, monocytes and macrophages are the three major types of phagocytes.

An important result of inflammation is the recruitment of phagocytes. These cells function to engulf and attack particles in the host that have been signaled for removal by various mechanisms. In this section we describe the synthesis and properties of phagocytes.

Phagocytes originate in the bone marrow from stem cells that first differentiate into myeloid precursor cells. These are a set of intermediate cells that can take a number of paths. Under the influence of various cytokines, the myeloid precursor cells then differentiate into different kinds of phagocytic cells such as polymorphonuclear granulocytes, monocytes and macrophages as shown in Figure 15-11. Polymorphonuclear granulocytes (mostly neutrophils) leave the bone marrow and circulate in the bloodstream and body for only a few days before dying. They are the first phagocytic cells that normally encounter an infection and can rise to large numbers during a severe illness. Monocytes differ from polymorphonuclear granulocytes in that they have an unsegmented nucleus and are much longer lived. This longer existence is critical to their function, as you will see later in the chapter. After creation in the bone marrow, they circulate in the blood for a period before settling in a tissue and maturing into macrophages. Finally, macrophages are mature monocytes that are attached to lymph tissues and can be up to ten times the size of monocytes. All phagocytes contain membrane vesicles filled with destructive and degradative compounds (lysosomes) that are available for the annihilation of engulfed microbes.

Phagocytosis

Phagocytes are motile and roam throughout the bloodstream, the lymphatic system and non-vascular tissue in search of particles to assault. When a non-self particle is encountered, it is taken into the phagocyte, combined with destructive compounds and destroyed. This process, termed phagocytosis, involves several discrete steps as listed below and diagrammed in Figure 15-12.

1. Detection of the foreign particle and movement of the phagocyte to the area.
2. Attachment of the foreign particle to the phagocyte.
3. Engulfment or ingestion of the foreign particle into a vesicle called a phagosome.
4. Fusion with lysosome and formation of the phagolysosome.
5. Intracellular killing and digestion.
6. In the case of macrophages, egestion and antigen presentation.

For the sake of simplicity, we will focus on the example of the attack of a phagocyte on a microorganism. However, the process is similar when a phagocyte attacks a virus or other foreign particle.

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