T Helper Cells and Immune Response

T Helper Cells and Their Function

• CD4 Cells: T helper cells are also known as CD4 cells. The differentiation between these terms is not critical for this course. Both terms refer to the same type of cell.

Antigen Presenting Cells (APCs)

• Definition: APCs are specialized cells (like dendritic cells or B cells) that present antigens to T cells.
• Process:
  • APCs ingest foreign antigens via endocytosis.
  • These antigens are then presented on the surface using Major Histocompatibility Complex (MHC) Class II molecules.

T Cell Activation Process

• Antigen Recognition: T helper cells have specific T cell receptors (TCRs) that recognize the foreign antigen presented by the APC on MHC Class II.
  • APC Interaction: For T helper cells to bind to APCs, a CD4 protein serves as an anchor to bind specifically with MHC Class II proteins.
• Antigen Binding: T cells can only respond to antigens that have been processed and presented. This means that T cells don't respond to free antigens as B cells do.

Steps of T Cell Activation

1. Antigen Binding: The first step is the T cell receptor recognizing and binding to the antigen presented on the MHC II molecule of the APC.
2. Costimulation: The second necessary step involves stimulation by other signals, primarily through cytokines from the APC.
   • Cytokines: IL-1 is released by APCs to costimulate CD4 cells, initiating the activation process.
   • After initial activation, the T helper cells produce IL-2, which provides further costimulation to CD8 cells and T helper cells.
3. Clonal Expansion: After costimulation, the CD4 cells proliferate and differentiate into effector T cells and memory T cells.
   • Effector T cells are responsible for performing functions in the immune response, while memory cells remain for future responses.

Role of CD4 T Cells in Immune Response

• Center of Immune Response: CD4 T cells are critical for the initiation of immune responses, as they provide necessary signals to activate CD8 T cells and B cells. Without CD4 cells, an immune response cannot occur.
• HIV/AIDS Connection:
  • HIV: Human Immunodeficiency Virus specifically targets and destroys CD4 T cells, leading to a weakened immune system.
  • AIDS: Acquired Immunodeficiency Syndrome describes a condition where the immune system is severely compromised due to low levels of CD4 cells.

Cytotoxic T Cells (CD8)

• Role: Cytotoxic T cells recognize and kill infected or cancerous cells presenting endogenous antigens via MHC Class I.
  • The receptor for CD8 T cells also binds to MHC Class I but does not involve CD4.
• Mechanism:
  • Following proper antigen and co-stimulation, CD8 cells proliferate into effector cells which release perforin and granzymes that lead to the death of infected or transformed cells through apoptosis.

Summary of Cellular Interactions

• Comparison of T Cell Activation:
  • T helper Cell (CD4):
    • Binds exogenous antigens on MHC-II (via CD4 protein).
    • Requires costimulation from IL-1 to initially activate, followed by IL-2 from cloned cells.
  • Cytotoxic T Cell (CD8):
    • Binds endogenous antigens on MHC-I (via CD8 protein).
    • Primarily costimulated by IL-2 from activated T helper cells.

Conclusion

• Understanding the role of CD4 T cells and the costimulation process is crucial for comprehending how the immune system functions and responds to infections and diseases, including the implications of viral infections like HIV that specifically target these cells.