Comprehensive Notes on the Cellular Immune System

Immune System Overview

Introduction

  • The immune system defends against infection and disease.
  • It consists of specialized cells working together.
  • This video introduces the major cellular components.

Two Arms of the Immune System

  • Innate Immunity:
    • First line of defense.
    • General immune response.
    • Classic symptoms include fever and inflammation.
  • Adaptive Immunity:
    • Specific detection and elimination of pathogens.
    • Slower to activate than innate immunity.
    • Provides immunological memory: immune cells remember previous infections for quicker future clearance.

Origin of Immune Cells

  • All immune cells originate from hematopoietic stem cells in the bone marrow.
  • Hematopoietic stem cells differentiate into:
    • Lymphoid progenitors
    • Myeloid progenitors
  • Lymphoid lineage (top).
  • Myeloid lineage (bottom).
  • Myeloid progenitors also produce red blood cells and platelets (not the focus here).

Lymphoid Lineage

  • Lymphoid progenitor differentiates into:
    • B cells
    • Natural killer cells
    • T cells
B Cells
  • Mature B cells differentiate into:
    • Memory cells
    • Plasma cells
  • Plasma cells secrete antibodies (important for adaptive immunity).
Natural Killer (NK) Cells
  • Cytotoxic cells of the innate immune system.
  • Detect and kill virus-infected cells.
T Cells
  • Arise from a common progenitor.
  • Types include:
    • Memory T cells: Faster immune response in future infections.
    • Cytotoxic T cells: Recognize antigens on infected cells and kill pathogen-specifically.
    • Helper T cells: Secrete cytokines to boost the adaptive immune response; activate B cells.

Myeloid Lineage

  • Produces most cells of the innate immune system.
  • Produces antigen-presenting cells (APCs) that prime adaptive immunity.
  • Myeloid progenitor gives rise to:
    • Neutrophils: Innate immune cells that capture and kill microorganisms.
    • Eosinophils: Granulocytes that release cytokines against parasites.
    • Monocytes: Differentiate into dendritic cells and macrophages.
    • Dendritic cells: Phagocytic cells bridging innate and adaptive immunity.
    • Macrophages: Tissue-resident phagocytic cells that patrol the body, clean up infection, and activate other immune cells.
    • Mast cells: Granulocytes implicated in allergy.
    • Basophils: Involved in the immune response to parasites.

Immune Cell Activation

  • Innate Immune Activation:
    • Cells express pattern recognition receptors (PRRs) on their surface.
    • PRRs bind to pathogens or pathogen components.
    • Binding induces intracellular signals that activate innate immune response.
    • Recognized particles are common among pathogens.
  • Adaptive Immune Activation:
    • Cells present antigen (small peptide fragments of pathogens) to T cells.
    • Antigen presentation informs a specific immune response.
    • Antigen is presented by:
      • MHC class one
      • MHC class two
MHC Class One
  • Expressed by all cells in the body.
  • Defense against intracellular pathogens (e.g., viruses).
  • Presents endogenous (intracellular) antigens to cytotoxic T cells.
MHC Class Two
  • Presents exogenous antigen (found on pathogens outside of cells).
  • Activates helper T cells.
  • Expressed by professional antigen-presenting cells (APCs):
    • Dendritic cells
    • Macrophages
    • B cells

Safeguards

  • Safeguards are in place to prevent the immune system from attacking self-antigens.

Conclusion

  • The mammalian immune system is complex and involves cells that contribute to innate and adaptive immunity.
  • This video provides a foundational understanding for further exploration.