2.5.3 Phagocytosis

Steps in Phagocytosis:

1. Detection and Binding:

• Phagocytes (e.g., macrophages, neutrophils) detect pathogens by recognising their antigens.
• The phagocyte binds to the pathogen using receptors on its cell surface membrane.

2. Engulfment:

• The phagocyte extends its cytoplasm, surrounding the pathogen and forming a phagocytic vacuole (also known as a phagosome).

3. Fusion with Lysosome:

• The lysosome (an organelle containing digestive enzymes) fuses with the phagocytic vacuole.
• This forms a phagolysosome.

4. Digestion:

• Lysosomal enzymes (e.g., lysozymes) break down the pathogen into smaller, harmless molecules.

5. Antigen Presentation:

• Fragments of the pathogen, including its antigens, are transported to the surface of the phagocyte.
• The phagocyte becomes an antigen-presenting cell (APC), displaying the pathogen's antigens to other immune cells.

6. Activation of Immune Response:

• The antigen-presenting phagocyte interacts with T lymphocytes, activating the specific immune response.

Key Features of Phagocytosis:

• Non-specific defence mechanism:
  • Phagocytosis targets any foreign material, not just specific pathogens.
• Role in immunity:
  • Removes pathogens and presents antigens to T cells, bridging the innate and adaptive immune responses.

Types of Phagocytes:

1. Macrophages:

• Large phagocytes that reside in tissues and are long-lived.
• Act as antigen-presenting cells to activate T cells.

2. Neutrophils:

• Small, short-lived phagocytes that circulate in the blood.
• Rapidly respond to infections and are abundant at infection sites.

Importance of Phagocytosis:

1. Pathogen Removal:

• Directly eliminates harmful microorganisms from the body.

2. Activation of Specific Immunity:

• Antigen presentation stimulates T cells, leading to a targeted immune response.

3. Inflammatory Response:

• Phagocytosis triggers the release of cytokines, which attract more immune cells to the infection site.