Adaptive Immune System Part 2

Adaptive Immune System - Part 2

• The adaptive immune system is activated when the innate immune system is overwhelmed.
• It involves two main branches: T cell-mediated and antibody-mediated (humoral).
• T cell activation requires antigen presentation.
• B cell activation occurs without the need for antigen presentation.

T-Cell Maturation and Activation Mechanisms

• T cells originate in the bone marrow.
• Immature T cells (thymocytes) migrate to the thymus for maturation.
• T-cell maturation involves:
  • Rearrangement of the T-cell receptor (TCR).
  • Positive selection: Thymocytes binding to MHC I or II molecules survive.
  • Negative selection: Thymocytes binding strongly to self-peptides or MHC complexes undergo apoptosis.
• Mature T cells then migrate to lymph nodes.

T-Cell Functions

• CD8 T cells (Tc) recognize antigens on MHC class I molecules and kill infected cells via apoptosis.
• CD4 T cells (Th) recognize antigens on MHC class II molecules.
  • Th1 cells activate macrophages and produce IFN-γ
  • Th2 cells activate eosinophils and mast cells, & support B cell antibody production.

CD4 T Cell Subsets and Functions

• Th1 cells produce IFN-γ, which activates macrophages, cytotoxic T cells, and other cells
• Th2 cells support antibody production by B cells, which is crucial for defense against pathogens that are outside of cells.
• Th17 cells produce IL-17, which promotes inflammation and neutrophil activity, which is important in defense against extracellular bacteria and fungi.
• Treg cells (regulatory T cells) suppress immune responses, which is important in preventing autoimmune diseases and limiting excessive inflammation.

Professional Antigen-Presenting Cells (APCs)

• APCs are crucial for T cell activation.
• Professional APCs, such as dendritic cells, macrophages, and B cells present antigens on their MHC class I and II molecules.
• Dendritic cells, macrophages, and B cells have different roles and characteristics.

Antigen Cross-Presentation

• Dendritic cells can cross-present antigens, meaning they can present intracellular pathogens on MHC class I and II molecules.
• This process triggers activation of both CD4+ and CD8+ T cells.

T-Cell Receptor (TCR) Structure

• The TCR is a membrane-bound glycoprotein with two polypeptide chains (alpha/beta or gamma/delta).
• TCR regions include variable (V) and constant (C) regions.
• Recognizes antigens presented on host cells (either MHC class I or MHC class II)

T-Cell Activation Mechanisms

• Specific antigen-MHC complex interaction with TCR.
• Supporting interactions from CD4 or CD8 co-receptors.
• Intracellular signaling and stabilization of the MHC-TCR connection.
• Co-stimulation from professional APCs like dendritic cells, macrophages, and B cells.

T-Cell Proliferation and Differentiation

• Activated T cells proliferate and differentiate into effector and memory T cells.
• Effector cells have short-lived and carry out immune functions directly
• Memory cells are long-lived, which provide protection against future infections.

Lymphocyte Activation and Differentiation (Summary)

• T-cell precursors undergo positive selection (cortex) and negative selection (medulla) in the thymus to become mature T cells.
• T-cell activation in lymph nodes involves antigen recognition and co-stimulation by professional APCs.
• Activated T cells differentiate into distinct effector (e.g., Th1, Th2) and memory subsets.

Summary of Adaptive Immune Response Phases:

• Antigen recognition by lymphocytes
• Lymphocyte activation
• Antigen elimination by effector cells
• Contraction (homeostasis) and memory development

Task 1: Immune System Parts (Template information)

• The document provides an example of a potential task where students need to fill an immune system template with specific criteria.

Task 2: Revision Questions

• The document suggests that revision questions will be answered at the end of the session.

Explore the workings of the adaptive immune system in this quiz focusing on T-cell maturation, activation mechanisms, and their functions. Understand how T cells respond to antigens and the critical processes involved in ensuring immune efficiency.