Steps of Phagocytosis

The process of phagocytosis occurs by following the steps given below:

Step 1: Phagocytic Cell Activation and Chemotaxis

Chemotaxis is the mechanism by which phagocytes are drawn to and migrate toward a range of chemicals produced during the immune response during the first stage of phagocytosis.

• Resting State: Phagocytes like macrophages, neutrophils, dendritic cells, and eosinophils patrol the body in a resting state.
• Activation Triggers: Exposure to bacterial components (e.g., lipopolysaccharides), cytokines released by immune cells, and complement proteins (parts of the immune system) activate these phagocytes.
• Enhanced Function: Activation increases their metabolic activity and ability to kill microbes. They also express more receptors on their surface to better recognize and bind to targets.

Step 2: Identifying Encroaching Bacteria

• Phagocyte Receptors: Phagocytes have various receptors that recognize specific molecules on pathogens (viruses, bacteria) or debris from dead cells.
  • Pattern Recognition Receptors (PRRs): These identify general danger signals like bacterial DNA, flagellin, or lipopolysaccharides.
  • Fc receptors: These bind to the Fc portion of antibodies attached to microbes, enhancing recognition (opsonization).
  • Complement Receptors: These bind to complement proteins deposited on pathogens by the complement system, another part of the immune response.
• Adhesion: Binding of these receptors to target molecules triggers the formation of membrane protrusions called pseudopods that engulf the microbe.

Stage 3: Ingestion and Arrangement of Phagosomes

• Following attachment, actin filaments undergo polymerization and subsequently depolymerization, which causes pseudopods to emerge and engulf the bacterium.
• The material is contained within a phagosome, an endocytic vesicle, which is subsequently transported to the endocytic processing pathway, following the fusion of the pseudopodia.

Step 4: Phagolysosome Formation

A phagosome travels along this pathway and eventually merges with a lysosome to produce a phagolysosome inside the cell.

Step 5: Formation of Remnant Bodies and Microbial Killing

Lysosomes have lysozyme as well as a number of cytotoxic and antibacterial compounds that can kill phagocytosed cells and germs. Either oxygen-dependent or oxygen-independent techniques are used to destroy microorganisms.

Oxygen-Dependent Killing:

1. Reactive Oxygen Intermediates (ROIs): Activated phagocytes produce large amounts of ROIs and reactive nitrogen intermediates, which have strong antibacterial properties.
2. Respiratory Burst: This metabolic process activates membrane-bound oxidase, leading to the production of superoxide anion, hydroxyl radicals, and hydrogen peroxide.
3. Additional Compounds: Inside the phagolysosome, phagocytes produce nitric oxide, hypochlorite, and other powerful antibacterial compounds. These compounds are effective against bacteria, fungi, parasitic worms, and protozoa.

Oxygen-Independent Killing:

1. Hydrolytic Enzymes: Phagocytes produce enzymes such as cathepsin G, elastase, collagenase, cathelicidins, and bactericidal permeability-increasing protein, which break down germs without the need for oxygen.
2. Defensins: These are cytotoxic and antibacterial peptides produced by active macrophages. Defensins can destroy a variety of microorganisms, including Staphylococcus aureus, Streptococcus pneumoniae, Escherichia coli, Pseudomonas aeruginosa, and Haemophilus influenzae.
3. Tumor Necrosis Factor (TNF-α): Activated macrophages also secrete TNF-α, a cytokine with multiple functions, including cytotoxicity for certain tumor cells.

Step 6: Exocytosis or Elimination

Exocytosis is the process by which the phagolysosome’s digested contents are expelled.

Phagocytosis is a process by which certain cells engulf and digest large particles or microorganisms. Understanding the steps of phagocytosis in order helps explain how cells engulf and destroy harmful particles. In this article, we will learn about phagocytosis and its function.