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Lecture Notes 17

General Class Information

  • Final exam is scheduled for April 17, from 8 to 10 AM in ISEC 102.
  • Students should avoid making plans to leave campus before the exam.
  • The confusing concepts question will remain open for 24 hours after activation to earn participation credit.

Lecture 17 Overview: The Immune System

Focus: Adaptive Immunity (Part 2)

  • T cells: Linking humoral and cell-mediated immunity
  • MHC molecules and antigen-presenting cells

Topics Covered:

  1. HIV Virus and AIDS

    • Stages of HIV Infection:
    1. Primary: Non-specific flu-like symptoms
    2. Clinical latency: Decline in T helper (Th) cells
    3. Early symptomatic disease: High fever, unusual infections, some cancers
    4. AIDS: T cell count < 200 cells/µl, associated with unusual cancer and infections.

  2. Clostridioides difficile (C. diff)

    • Non-member of the Enterobacteriaceae family, causes colitis and life-threatening CDI.
    • Risk is higher for patients on antibiotics.
    • Produces cytotoxins leading to pseudomembranous colitis.

  3. Sepsis

    • Often caused by bacterial infections.
    • Characterized by SIRS symptoms:
      • Abnormal WBC count
      • Abnormal body temperature
      • Rapid heart/breathing rates
    • Considered a medical emergency.

  4. Antigen-Presenting Cells (APCs)

    • Engulf microbes, digest, and present antigens for T cell activation.
      • MHC Class I: Present on all nucleated cells (absent in RBCs)
      • MHC Class II: Found on professional APCs (e.g., dendritic cells, macrophages, B cells).

T Cells: Types and Activation

  • Main Types of T Cells:
    • Cytotoxic T Cells (TC): CD8 receptors; kill infected host cells.
    • T Helper Cells (TH): CD4 receptors; help activate B cells and TC cells.
  • Activation Process:
    • T cells require antigen presentation by APCs via MHC molecules.
    • TCR (T-cell receptor) binds antigens within MHC molecules.

MHC Molecules and Antigen Processing

  1. MHC Class I Presentation:

    • Present intracellular pathogens.
    • Antigens from degraded proteins in the cytoplasm.

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    • Pathogen proteins degraded
    • Antigens presented on MHC I on cell surface.

  2. MHC Class II Presentation:

    • Present extracellular pathogens.
    • Pathogens phagocytosed and digested.

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    • Antigens loaded onto MHC II from the endosome for presentation.

T Cell Activation Process

  • Signals Required for Activation:

    1. Antigen-MHC binding (TCR to MHC)
    2. Second signal (CD28 on T-cell binds B7 on APC)
    3. Cytokine signaling from TH cells.

  • Effects of Activation:

    • Clonal expansion of effector and memory TC cells.
    • Effector TC cells move to infection sites and destroy infected cells.

Case Study: Bare Lymphocyte Syndrome (BLS)

  • Symptoms: Frequent infections due to low IgG, IgA, and IgM levels.
  • Test results: Absence of MHC Class II proteins, affecting TH cell activation and antibody response.
  • Treatment: Bone marrow transplant recommended due to defective antigen presentation.

Summary of Innate vs. Adaptive Immunity

  • The adaptive immune response provides capability for stronger and faster responses upon re-encounter with pathogens and includes memory formation crucial for long-term immunity.