Makindo Medical Notes

Related Subjects: |X linked Agammaglobulinaemia (Bruton) |X-linked lymphoproliferative disease (Children) |Chediak Higashi syndrome |Common variable immunodeficiency |Severe combined immunodeficiency disorders |DiGeorge syndrome (thymic aplasia) |Selective IgA deficiency |Wiskott-Aldrich syndrome (Children) |T lymphocytes

Overview of T Lymphocytes

T lymphocytes, or T cells, are a type of white blood cell that play a central role in the adaptive immune response. They are essential for identifying and eliminating infected cells, coordinating the immune response, and maintaining immune system memory.

Types of T Lymphocytes

• Helper T Cells (CD4⁺ T Cells):
  • Assist other immune cells by releasing cytokines.
  • Subsets include:
    • Th1 Cells: Promote cell-mediated immunity and are involved in the response to intracellular pathogens.
    • Th2 Cells: Support humoral immunity by stimulating B cells to produce antibodies, primarily against extracellular pathogens.
    • Th17 Cells: Involved in inflammation and defense against extracellular bacteria and fungi.
    • Treg Cells: Regulate immune responses and maintain tolerance to self-antigens.
• Cytotoxic T Cells (CD8⁺ T Cells):
  • Directly kill infected or cancerous cells by inducing apoptosis.
• Memory T Cells:
  • Provide long-term immunity by remembering past infections and responding more rapidly upon re-exposure to the same antigen.
• Natural Killer T (NKT) Cells:
  • Bridge innate and adaptive immunity by recognizing lipid antigens presented by CD1d molecules.
• Gamma Delta (γδ) T Cells:
  • Possess a distinct T cell receptor (TCR) and are involved in both innate and adaptive immune responses.

T Cell Development and Maturation

• Origin:
  • T cells originate from hematopoietic stem cells in the bone marrow.
• Thymic Education:
  • T cells migrate to the thymus, where they undergo selection processes:
    • Positive Selection: Ensures T cells can recognize self-MHC molecules.
    • Negative Selection: Eliminates T cells that strongly react to self-antigens, preventing autoimmunity.
  • Mature T cells express either CD4 or CD8 co-receptors and a functional T cell receptor (TCR).

T Cell Activation

• T cell activation requires two signals:
  • Signal 1: TCR recognition of the antigen-MHC complex on the surface of antigen-presenting cells (APCs).
  • Signal 2: Co-stimulatory signals provided by interactions between co-stimulatory molecules on the APC and the T cell (e.g., CD28 binding to B7).
• Upon activation, T cells proliferate and differentiate into effector T cells to carry out their immune functions.

Functions of T Lymphocytes

• Helper T Cells (CD4⁺):
  • Coordinate the immune response by releasing cytokines that influence the activity of other immune cells (e.g., macrophages, B cells, and cytotoxic T cells).
• Cytotoxic T Cells (CD8⁺):
  • Directly kill infected or cancerous cells by inducing apoptosis through the release of perforin and granzymes.
• Regulatory T Cells (Treg):
  • Suppress immune responses to prevent autoimmunity and maintain immune homeostasis.
• Memory T Cells:
  • Provide long-term immunity by remaining in the body after an infection has been cleared, enabling a faster and stronger response upon re-exposure to the same pathogen.

Clinical Relevance

• Autoimmune Diseases:
  • Occur when T cells mistakenly attack self-tissues. Examples include multiple sclerosis, rheumatoid arthritis, and type 1 diabetes.
• Immunodeficiency:
  • Conditions where T cell function is impaired, leading to increased susceptibility to infections. Examples include HIV/AIDS and severe combined immunodeficiency (SCID).
• Transplantation:
  • T cells play a role in graft rejection. Immunosuppressive drugs are often used to prevent this response.
• Cancer Immunotherapy:
  • Strategies such as CAR-T cell therapy involve engineering T cells to target and kill cancer cells.

Summary

T lymphocytes are essential components of the adaptive immune system, involved in recognizing and responding to pathogens, coordinating immune responses, and maintaining long-term immunity. Their development, activation, and function are tightly regulated to ensure effective immune responses while preventing autoimmunity. Understanding T cell biology is crucial for diagnosing and treating various immune-related diseases and developing advanced immunotherapies.