T Cell Immunology

Key Concepts of T Helper Cells and Cytotoxic T Cells

T Helper Cells (CD4 Cells)

• Definition: T helper cells, also known as CD4 cells, are crucial players in the immune response.

• Function: They help other immune cells by producing signaling molecules (cytokines) that stimulate the immune response.

• Activation:

  • T helper cells are activated by recognizing antigens presented on MHC II molecules by Antigen Presenting Cells (APCs).

  • The antigen presenting cell (APC) can be a dendritic cell or a B cell.

  • The antigen is endocytosed and presented on the surface of the APC through MHC II.

Diagram of Activation Process

• Antigen Binding:

  • The T cell receptor (TCR) on the T helper cell binds to the MHC II-antigen complex on the APC. This is essential for T helper cell activation.

  • The CD4 protein on T cell acts as an anchor, binding specifically to MHC II.

• Co-stimulation:

  • Following the initial binding, two crucial steps for T cell activation occur:

  1. Antigen Binding: The TCR binds to the antigen presented on MHC II.

  2. Co-stimulation: The APC secretes interleukin-1 (IL-1), providing necessary signals for T cell activation. This leads to the production of more IL-2 by the CD4 T cell, which further stimulates itself and other cells.

Cloning of T Helper Cells

• Results in the formation of:

  • Effector T cells: Produce more cytokines, such as IL-2, to facilitate immune response.

  • Memory T cells: Provide a quicker immune response upon re-exposure to the same antigen.

Cytotoxic T Cells (CD8 Cells)

• Definition: Cytotoxic T cells, also known as CD8 cells, target and kill infected or cancerous cells.

• Activation Process:

  • Similar to T helper cells, but they bind to endogenous antigens presented on MHC I.

  • CD8 protein on cytotoxic T cells interacts specifically with MHC I, allowing effective binding and response to infected cells.

  • Like T helper cells, cytotoxic T cells require co-stimulation, often provided by cytokines from the helper T cells.

Mechanism of Action

• Upon activation, cytotoxic T cells produce perforin and granzymes:

  • Perforin helps form pores in the target cell membrane.

  • Granzymes enter through those pores and induce apoptosis (cell death) in infected or cancerous cells.

Importance of CD4 T Helper Cells

• T helper cells are central coordinators in the immune response:

  • Without CD4 cells, there is no sufficient activation of B cells or cytotoxic T cells, which severely impairs the adaptive immune response.

  • The absence of CD4 cells is prominent in HIV/AIDS, where the virus specifically attacks these helpers, leading to a compromised immune system.

Key Terms

• MHC (Major Histocompatibility Complex): Molecules on cell surfaces displaying antigens to immune cells.

• Cytokines: Signaling proteins that mediate and regulate immunity, inflammation, and hematopoiesis.

• Co-stimulation: The second signal that T cells need for full activation, involving multiple receptor interactions and cytokine release.

Clinical Relevance

• HIV/AIDS: A virus that targets CD4 T helper cells preventing effective immune response and leaving individuals vulnerable to infections and certain cancers due to lack of proper T cell activation.