Phagocytosis

• What are secondary defences?

  They target pathogens that breached primary defences.
• What are the two levels of the immune response?

  1. Non-specific response: Phagocytosis occurs regardless of pathogen.
  2. Specific response: B and T lymphocytes recognize antigens and produce antibodies.
• What are antigens?

  Molecules on cell surfaces specific to organisms.
• What triggers the immune response?

  • Foreign antigens trigger the immune response.
• What are phagocytes?

  Non-specific white blood cells that digest foreign particles.
• What are the two types of phagocytes?

  • Neutrophils
  • Macrophages
• Where are macrophages made?

  In the bone marrow.
• How do macrophages develop?

  They circulate as monocytes and settle in lymph nodes.
• How does histamine help macrophages fight infection?

  • Histamine makes capillaries leaky.
  • Increases tissue fluid in lymph nodes.
  • Helps deliver pathogens to macrophages.
• What role do macrophages play in the specific immune response?

  They act as antigen-presenting cells to activate lymphocytes.
• How do macrophages present antigens?

  They attach antigens to MHC proteins after engulfing pathogens.
• Why is antigen presentation important?

  It helps the immune system recognize pathogens.
• What does the MHC complex do?

  Prevents APCs from being mistaken as foreign cells.
• What are neutrophils?

  A type of short-lived phagocyte that digests pathogens.
• What percentage of white blood cells are neutrophils?

  60% of white blood cells.
• Describe the nucleus of a neutrophil.

  It has a tri-lobed nucleus.
• Where are neutrophils made and found?

  • Made in the bone marrow.
  • Found in blood, tissue fluid, and epithelial surfaces.
• When are neutrophils released in large numbers?

  In response to infection.
• How do neutrophils find infected cells?

  They are attracted by histamine released from infected cells.
• What is opsonisation?

  The process where opsonins coat bacteria for phagocytosis.
• What role do antibodies play in phagocytosis?

  They bind to antigens, marking pathogens for destruction.
• What is chemotaxis?

  The movement of phagocytes towards pathogens.
• What is a phagosome?

  A vesicle inside the phagocyte that traps the pathogen.
• How do lysosomes help destroy pathogens?

  Lysosomes fuse with phagosomes and release lysin enzymes.
• How are waste products removed after phagocytosis?

  Waste is excreted from the phagocyte by exocytosis.
• What is phagocytosis?

  The process by which phagocytes engulf and destroy pathogens.
• What are opsonins?

  Plasma proteins or antibodies that mark pathogens.
• What is a lysosome?

  A membrane-bound organelle with digestive enzymes.
• What is a phagosome?

  A vesicle inside a phagocyte that traps pathogens.
• Why are phagocytes considered a secondary defence?

  • They act after pathogens breach primary defences.
• What are the key differences between neutrophils and macrophages?

  | Feature | Neutrophils | Macrophages |

  |---------|------------|-------------|

  | Lifespan | Short-lived | Long-lived |

  | Percentage of WBCs | 60% | Less common |

  | Location | Blood, tissues, epithelial surfaces | Lymph nodes, tissues |

  | Function | Rapid response, phagocytosis | Antigen presentation, immune activation |

  | Nucleus Shape | Tri-lobed | Large, round |
• How do macrophages act as antigen-presenting cells (APCs)?

  1. They engulf a pathogen but do not fully digest it.
  2. They attach antigens to MHC proteins.
  3. The antigen-MHC complex is displayed on the macrophage’s surface.
  4. This activates lymphocytes, triggering a specific immune response.
• Why is antigen presentation important?

  • Allows B and T lymphocytes to recognize pathogens.
  • Helps trigger the specific immune response.
  • Prevents the immune system from attacking the macrophage itself.
• Describe how phagocytosis occurs.

  1. Opsonisation – Opsonins coat the pathogen.
  2. Chemotaxis – Phagocyte moves towards the pathogen.
  3. Attachment – Phagocyte binds to the pathogen.
  4. Engulfment – Pathogen is engulfed into a phagosome.
  5. Digestion – Lysosomes fuse with the phagosome, releasing enzymes.
  6. Absorption – Nutrients and waste products are absorbed or excreted.