BIO221 Mechanism of Phagocytosis

Phases of Phagocytosis Steps

1. Chemotaxis & Adherence 2. Ingestion 3. Formation of Phagosome 4. Fusion of Phagosome with a lysosome to form a phagolysosome 5. Digestion 6. Formation of Residual Body which is discharged from the phagocyte

Formation of Residual Body which is discharged from the phagocyte

After enzymes have digested the contents of the phagolysosome brought into the cell by ingestion, the phagolysosome contains indigestible material and is called a residual body. This residual body then moves toward the cell boundary and discharges its wastes outside the cell

Adherence

As it pertains to phagocytosis, adherence is the attachment of the phagocyte's plasma membrane to the surface of the microorganism or other foreign material. Adherence is facilitated by the attachment of pathogen associated molecular patterns (PAMPs) of microbes to receptors, such as Toll-like receptors (TLRs), on the surface of phagocytes. The binding of PAMPs to TLRs not only initiates phagocytosis, but also induces the phagocyte to release specific cytokines that recruit additional phagocytes

Chemotaxis

Chemotaxis is the chemical attraction of phagocytes to micro organisms. Among the chemotactic chemicals that attract phagocytes are microbial products, components of white blood cells and damaged tissue cells, cytokines released by other white blood cells, and peptides derived from complement

Opsonization

Coating process that enhances Phagocytosis

Ingestion

Following adherence, ingestion occurs. During this process, the plasma membrane of the phagocyte extends projections called pseudopods that engulf the microorganism

Fusion of Phagosome with a lysosome to form a phagolysosome

In this phase of phagocytosis, the phagosome pinches off from the plasma membrane and enters the cytoplasm. Within the cytoplasm, it contacts lysosomes that contain digestive enzymes and bactericidal substances. On contact, the phagosome and lysosome membranes fuse to form a single, larger structure called a phagolysosome. The contents of the phagolysosome brought in by ingestion are digested in the phagolysosome

Digestion

Lysosomal enzymes that attack microbial cells directly include lysozyme, which hydrolyzes peptidoglycan in bacterial cell walls. A variety of other enzymes, such as lipases, proteases, ribonuclease, and deoxyribonuclease, hydrolyze other macromolecular components of microorganisms. Lysosomes also contain enzymes that can produce toxic oxygen products such as superoxide radical (O2-∙), hydrogen peroxide (H2O2), nitric oxide (NO), singlet oxygen (1O2−), and hydroxyl radical (OH·). Toxic oxygen products are produced by a process called an oxidative burst. Other enzymes can make use of these toxic oxygen products in killing ingested microorganisms. For example, the enzyme myeloperoxidase converts chloride (Cl−) ions and hydrogen peroxide into highly toxic hypochlorous acid (HOCl). The acid contains hypochlorous ions, which are found in household bleach and account for its antimicrobial activity

Formation of Phagosome

Once the microorganism is surrounded, the pseudopods meet and fuse, surrounding the microorganism with a sac called a phagosome,or phagocytic vesicle.The membrane of a phagosome has enzymes that pump protons (H+) into the phagosome, reducing the pH to about 4. At this pH, hydrolytic enzymes are activated