Notes on Adaptive Immunity and T Lymphocytes

Adaptive Immunity Overview

  • Definition: Adaptive immunity is the third line of defense in the immune system and is characterized by:

    • Antigen Specificity: Targets specific pathogens or antigens.

    • Systemic Response: Involves the entire body, not just localized areas.

    • Memory: Provides a faster and stronger response upon re-exposure to the same pathogen.

Mechanisms of Adaptive Immunity

  • Humoral Immunity:

    • Carried out by B lymphocytes (B cells).

    • Involves the production of antibodies that neutralize pathogens.

  • Cellular Immunity:

    • Carried out by T lymphocytes (T cells).

    • Targets infected cells, cancer cells, and foreign cells.

Key Terms and Definitions

  • Antigens: Substances that provoke an immune response; known as antibody generators.

  • Self Antigens: Host proteins recognized by the immune system as a part of the body; includes Major Histocompatibility Complex (MHC) proteins.

Key Types of Immune Cells

  1. B Lymphocytes (B cells): Responsible for humoral immunity.

  2. T Lymphocytes (T cells):

    • Involved in cellular immunity.

    • Include:

      • Helper T Cells (CD4): Activate other immune cells.

      • Cytotoxic T Cells (CD8): Kill infected or cancerous cells.

  3. Antigen Presenting Cells (APCs):

    • Macrophages that digest pathogens and present antigens on their surface to T cells.

T Cell Maturation Process

  1. Origin: All white blood cells originate in the bone marrow.

  2. Migration to Thymus: Immature T cells leave the bone marrow to mature in the thymus, a lymphoid organ critical for immune system development.

  3. Selection Process:

    • Thymus screens T cells, eliminating self-reactive T cells (95% die during this process).

    • Only T cells that can recognize foreign antigens survive to circulate in the body.

Activation of T Cells

  • T Cell Activation:

    • Antigen presenting cells display antigen fragments on their MHC molecules.

    • MHC-antigen complex binds to specific T cell receptor.

    • T cell binds to costimulatory signal for activation, leading to proliferation and differentiation into effector and memory T cells.

Functions of Activated T Cells

Helper T Cells (CD4)

  • Directly stimulate B cell proliferation (antibody production).

  • Secrete cytokines (e.g., interleukin-2, interleukin-3) that:

    • Enhance macrophage activity.

    • Activate cytotoxic T cells.

    • Stimulate B cell activity.

Cytotoxic T Cells (CD8)

  • Mechanism of Action:

    1. Bind to target cells (e.g., infected, cancerous cells).

    2. Release perforin, which forms pores in the target cell membrane (functionally similar to the MAC of the complement system).

    3. Enzymes are released into the target cell through these pores, leading to the target's destruction.

    4. Detach and seek new target cells once the initial target is destroyed.

  • Summary Diagram:

    • Binding: Target + cytotoxic T cell.

    • Secretion: Perforin + enzymes.

    • Outcome: Target cell destruction.

Conclusion

  • This video sets the stage for further exploration of the B lymphocytes and their role in the adaptive immune response in subsequent discussions.