In-Depth Notes on T Lymphocytes and Their Functions

Overview of T Lymphocytes

  • T cells are a crucial part of the adaptive immune system.

Functions of T Cells

  • Cell-mediated immunity: T cells directly engage with infected or abnormal cells.
  • Powerful Coordination and Killing: Essential for targeting intracellular antigens (e.g., viruses).
  • Intercellular Communication: Organizes responses and mediates interactions with other immune cells, including B cells.

Types of T Cells

  • CD4+ T helper cells: Enhance the immune response by helping B cells and stimulating macrophages.
    • TH1: Activate macrophages and cytotoxic T cells, release cytokines like IFN-γ.
    • TH2: Activate B cells and help in immunoglobulin class-switching, releasing cytokines like IL-4 and IL-5.
  • CD8+ Cytotoxic T cells (CTLs): Directly kill infected host cells using mechanisms such as cytotoxic granules.

T Cell Activation Process

  1. Antigen Presentation: T cells are activated via interaction with antigen presenting cells (APCs) such as macrophages and dendritic cells.
    • MHC molecules: Antigen-MHC complex recognition is crucial; Type I MHCs (on nucleated cells) present intracellular antigens while Type II MHCs (on APCs) present extracellular antigens.
    • Co-stimulation Signals: Second signals (like CD28 binding to B7) are necessary for full activation.

T Cell Maturation and Development

  • Thymic Education: T cells mature in the thymus, undergoing positive and negative selection to ensure self-tolerance and functionality.
  • T Cell Receptors (TCR): Specific for one antigen, integral for recognizing MHC-antigen complexes.

Mechanisms of T Cells

  • Killing Mechanisms (CD8+ CTLs):
    • Perforin: Creates pores in infected cell membranes.
    • Granzymes: Induce apoptosis in target cells.
    • Fas Ligand: Interacts with target cell surface to induce apoptosis.

Regulatory T Cells

  • Act as suppressors of the immune response, maintaining tolerance to self-antigens and controlling inflammation.
  • Key Cytokines: Produce IL-10 and TGF-β to inhibit immune responses.

Clinical Relevance of T Cells

  • Autoimmune Diseases: Dysregulation of T cell functions can lead to autoimmune disorders.
  • Designer T Cells: Potential therapies involve engineering T cells to target specific diseases, such as cancers.

Conclusion**

  • T cells play complex roles in the immune system, mediating both direct actions (killing infected cells) and helping orchestrate overall immune responses. Their precise balance of activation and regulation is vital for maintaining health and preventing disease.