11.04 T-Lymphocytes (T cells)

Types of Lymphocytes

• Lymphocytes are a type of white blood cell essential for the adaptive immune response.
• They are smaller than phagocytes and characterized by a large nucleus.

Main Types of Lymphocytes

1. B-Lymphocytes (B Cells)
   • Maturation: Mature in the bone marrow.
   • Location: Primarily found in lymph nodes and the spleen.
   • Function: Each B cell is specific to a single antigen and can produce one type of antibody. They are crucial for humoral immunity, which involves antibody-mediated responses.
2. T-Lymphocytes (T Cells)
   • Maturation: Produced in the bone marrow and mature in the thymus gland, which is active until puberty and then gradually shrinks.
   • Function: Involved in cell-mediated immunity, with roles including directly destroying infected cells and coordinating the overall immune response.

T-Lymphocytes Detailed Overview

Origin and Development

• Production: Originate in the bone marrow.
• Maturation: Complete their development in the thymus.

T-Cell Receptors

• Structure: Similar to antibodies with specificity for antigens.
• Specificity: Each T-Cell Receptors are unique to a specific antigen, ensuring targeted immune responses.

Antigen Presentation

• Process: T cells recognize antigens presented on the surface of host cells (e.g., macrophages, infected cells) via Major Histocompatibility Complex (MHC) molecules.
• Activation: Recognition of antigens on host cells activates T cells to initiate appropriate immune responses.

Types of T-Lymphocytes

1. T-Helper Cells (CD4⁺ T Cells)
   • Function:
     • Release cytokines to coordinate the immune response.
     • Stimulate B cells to proliferate and differentiate into plasma and memory cells.
     • Enhance phagocytosis by macrophages.
   • Role: Act as regulators, ensuring that other immune cells respond effectively.
2. T-Killer Cells (Cytotoxic T Cells or CD8⁺ T Cells)
   • Function:
     • Identify and destroy infected or malignant cells presenting foreign antigens.
     • Induce apoptosis in target cells by releasing toxic substances such as perforins and granzymes.
   • Mechanism: Attach to infected cells, form pores in their membranes, and release enzymes that lead to cell death, eliminating pathogens within.

Key Processes Involving T Cells

1. Clonal Selection and Clonal Expansion
   • Clonal Selection: Only T cells with receptors specific to an encountered antigen are activated.
   • Clonal Expansion: Activated T cells proliferate through mitosis, increasing the number of cells available to fight the antigen.
2. Memory Cells Formation
   • Memory T-Helper and T-Killer Cells: Persist after the initial infection has been cleared, enabling a faster and more robust response upon subsequent exposures to the same antigen.

Key Terms

• Antigen Presentation: The display of antigen fragments on the surface of host cells via MHC molecules to alert and activate T cells.
• Cytokines: Signaling proteins released by T-helper cells that modulate the activity of other immune cells, including B cells, macrophages, and cytotoxic T cells.

T-Lymphocyte Response Process

1. Antigen Recognition
   • T-helper and T-killer cells bind to specific antigens presented on infected cells or antigen-presenting cells like macrophages.
2. Cytokine Release
   • T-Helper Cells:
     • Release cytokines to stimulate B cell proliferation and differentiation into plasma and memory cells.
     • Promote the division and activation of cytotoxic T cells.
   • Effect: Enhances the overall immune response by activating various immune cell types.
3. Killing Infected Cells
   • Cytotoxic T Cells:
     • Attach to infected cells.
     • Release toxins (e.g., perforins and granzymes) that create pores in the target cell membrane, leading to cell death and elimination of intracellular pathogens.
4. Formation of Memory Cells
   • Both T-helper and T-killer cells generate memory cells that remain in the body, ensuring a swift and effective response upon re-exposure to the same antigen.

Examples of Pathogen Responses

Corynebacterium diphtheriae Toxin Release

• Response: T-helper cells signal B cells to produce specific antibodies that neutralize the diphtheria toxin.

Measles Virus Invasion

• Response:
  • T-Killer Cells: Target and destroy virus-infected cells in the respiratory tract.
  • T-Helper Cells: Coordinate the production of antibodies by B cells to neutralize the virus.

Multiple Colds (Rhinovirus Infections)

• Challenge: Numerous rhinovirus strains present different antigens.
• Response: Each strain requires a separate primary immune response, leading to repeated infections as memory cells are specific to each antigen.