Stages Of White Cell Phagocytosis

The phagocytic process by which bacteria and other infectious agents are recognized and destroyed is a critical function of neutrophils and monocytes. The neu-trophil role in phagocytosis is localized and immediate; the monocyte role is related to immune response and more tissue oriented. The process by which bacteria are digested and immobilized can be broken down in several simplified steps (Fig. 10.1).

• Stage 1—CHEMOTAXIS: Foreign body invades tissues; neutrophils, which usually move in random motion through the tissue, are attracted directly to site of invasion through chemical signals sent by foreign body (bacteria). More neutrophils mobilize and rush to site of infection.

• Stage 2—OPSONIZATON: Neutrophilic attachment of the invading foreign body can only take place once the foreign body has been opsonized or prepared to be ingested through interaction with the complement system and other immunoglobulins.

• Stage 3—INGESTION: The opsonized foreign body is ingested by the neutrophil. The foreign body is engulfed by the neutrophilic pseudo-pod membranes.

• Stage 4—KILLING: The neutrophilic granules release their contents, which contain various lytic elements. The pH of the cell is reduced and hydrogen peroxide is produced by the neutrophil as a result of respiratory burst and released to accelerate the destruction process. The neutrophil is also destroyed in this process.

Bacteremia or sepsis may occur if invading organisms or foreign bodies are not destroyed upon entry into the body. They may locate in secondary sites such as the lymph nodes, where they will rapidly multiply and release toxins.

Phagocytic activity is a complex process involving phagocytic cells, the complement system, cytotoxins, and acute-phase reactants. Each of these systems must have coordinated activities to ensure that pathogens will be destroyed4 (Table 10.2).

Figure 10.1 Mechanism of phagocytosis. Stages depicted are (A) chemotaxis and directed motility, (B) opsonization, (C) ingestion, (D) degranulation and digestion.

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