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.