ADD ON TO INNATE IMMUNE SYSTEM

Overview of Immune System Responses

  • The immune system consists of multiple components, notably the innate and adaptive immunity.

  • Natural killer (NK) cells play a crucial role in recognizing infected or abnormal cells, specifically when these cells fail to present MHC class I molecules.

Key Concepts in Immunology

Major Histocompatibility Complex (MHC)

  • MHC Class I:
      - A molecule that presents intracellular proteins on the surface of cells.
      - Healthy cells present self-antigens, allowing the immune system to recognize normal cells.
      - Infected cells may downregulate MHC class I expression to evade detection by cytotoxic T cells.

Cytotoxic T cells vs. Natural Killer Cells

Cytotoxic T Cells

  • Role: Detect and kill infected cells through specific recognition of antigens presented by MHC class I.

  • Mechanism:
      - Cytotoxic T cells use their T-cell receptors (TCR) to identify infected cells displaying foreign antigens via MHC class I.

  • Action Upon Detection:
      - When a cytotoxic T cell encounters a matching pathogen, it induces apoptosis (death) in the infected cell.

Natural Killer Cells

  • Role: Provide a backup mechanism to identify and kill cells that do not present MHC class I molecules.

  • Detection Mechanism:
      - Natural killer cells assess the presence of MHC class I; absence signals potential infectivity.
      - They bind to activating molecules on target cells, determining the action based on MHC class I display.

  • Action When MHC Class I is Absent:
      - NK cells will engage the detected cell, inspect internal contents, and carry out cytotoxic activity (such as perforation of the cell membrane).

Role-Play Demonstration in Class

  • The lesson involved a role-play scenario to contrast actions of cytotoxic T cells and natural killer cells.

  • Participants:
      - A volunteer represented the cytotoxic T cell while another depicted the natural killer cell.

  • Activity:
      - The cytotoxic T cell checked for pathogens using TCRs. If a match was found, it triggered cell death.
      - The natural killer cell responded to cells lacking MHC class I, indicating potential infection or abnormality.

Recognition of Abnormal Cells

Pathogen Strategies

  • Some pathogens can inhibit MHC class I presentation through various mechanisms:
      - Disruption in transcription or translation of MHC class I proteins.
      - Sequestration or degradation of MHC class I molecules.

  • Cells with absent MHC class I trigger natural killer (NK) cells to investigate further and perform cytotoxic actions if necessary.

Special Considerations

Cells without MHC Class I

  • Red Blood Cells (RBCs):
      - Naturally lack MHC class I due to their lifespan and function.
      - Short lifespan does not typically necessitate MHC class I for self-identification.

  • Cancer Cells:
      - May exhibit mutations affecting MHC class I expression, evading detection by both cytotoxic T cells and natural killer cells.

Conclusion

  • Natural killer cells serve as a critical component of innate immunity, complementing adaptive responses by cytotoxic T cells.

  • Their ability to recognize cells lacking MHC class I offers a safeguard against undetectable infections and cancerous transformations.

  • Understanding the interplay between these immune components is vital for comprehending immune responses and developing therapeutic strategies in immunology.