Understanding B and T Lymphocytes in Immune Response

Introduction to Lymphocytes

• Presented by: Dr. Dona Wijayasinghe

• Special Thanks: Associate Professor Guna Karupiah

ILO 1: Describe how B and T lymphocytes recognise antigens

Recap: What are antigens?

• Antigens: Substances that induce an immune response.

• Antibody: A protein produced by B cells that binds to a specific antigen.

• Pathogen: A microorganism that can cause disease.

B Cell Recognition of Antigens

B Cell Receptors (BCR)

• Function: Recognise specific antigens on pathogens.

• Mechanism:
    - Pathogen antigens are attached to the B cell receptor (BCR).
    - Interaction leads to activation of B cells.

T Cell Recognition of Antigens

T Cell Receptors (TCR)

• Structure: Different receptors for CD4 Helper T cells and CD8 Cytotoxic T cells.

• Mechanism of Recognition:
    - Pathogen antigens are processed and presented by Antigen Presenting Cells (APCs).
    - For CD4 Helper T Cells:
      - Processed antigen presented on MHC Class II molecules from APCs (e.g., B cells).
    - For CD8 Cytotoxic T Cells:
      - Processed antigen presented on MHC Class I molecules from infected host cells.

• Markers:
    - CD4 for Helper T Cells.
    - CD8 for Cytotoxic T Cells.
  

Self-Recognition by T Cells

How do T Cells Recognise Self-Cells?

• T cells distinguish between infected cells and normal body cells using MHC markers:
    - MHC Class I for CD8 Cytotoxic T Cells.
    - MHC Class II for CD4 Helper T Cells.
  

ILO 2: Clonal Selection and Random Generation of Receptors

How are BCRs and TCRs generated?

• Both B and T cell receptors are specific to pathogens and created before encountering them.

• Process:
    - Somatic Recombination:
      - Different genes are shuffled to create diverse receptors.
    - Production:
      - Millions of unique receptors are created for both B and T cells.
      - CD4 Helper T Cells and CD8 Cytotoxic T Cells undergo similar processes.

Clonal Selection and Expansion

• Entry of Pathogen:
    - Pathogen's antigen binds to the specific receptor on B or T cells.
    - Activation occurs leading to:

• Clonal Selection:
    - The B cell or T cell with the matching receptor is activated.

• Clonal Expansion:
    - Proliferation of activated cells:
      - B Cells: Form memory B cells and plasma B cells.
      - CD4 Helper T Cells: Form memory T cells.
      - CD8 Cytotoxic T Cells: Form memory T cells.
  

ILO 3: B Lymphocytes Recognising and Tagging Pathogens for Destruction

Mechanism of B Lymphocyte Action

• Pathogen Encounter:
    - Recognition involves direct binding of the pathogen to BCRs.

• Stimulated B Cell: Initiates the immune response.

• Processes:
    - Clonal expansion enhances the immune response against the specific pathogen.
    - Differentiation into effector cells facilitates targeted destruction of pathogens.

ILO 4: CD4 Helper T Lymphocytes and MHC-II Responses

CD4 Helper T Cell Responses

• Respond to antigens presented by MHC Class II molecules from APCs.

• Pathogen Presentation:
    - Pathogen processed and displayed on the B cell or APC's surface.

• Activated CD4 Helper T Cell:
    - Undergoes clonal expansion to increase the immune response capacity.
    - Plays a crucial role in assisting B cells and cytotoxic T cells in pathogen elimination.

ILO 5: CD8 Cytotoxic T Lymphocytes and MHC-I Responses

CD8 Cytotoxic T Cell Responses

• Respond to MHC Class I molecules on infected cells.

• Activation Process:
    - Pathogen-infected cell presents processed antigen on MHC Class I.
    - CD8 T cells recognize this and become activated.

• Clonal Expansion:
    - Increases the number of cytotoxic T cells available to eliminate infected cells.

• Memory T Cells:
    - Some cells differentiate into memory T cells for long-lasting immunity.