Antigen Presentation

Lecture Overview

  • Lecture number six focusing on antigen presentation

  • Future topics include lymphocyte responses and the evolution of the immune system

  • Acknowledgement of significant contributions to immunology

Antigens and Lymphocyte Interaction

B Cells vs T Cells

  • B lymphocytes

    • Triggered by various antigens: proteins, sugars, lipids, nucleic acids, small peptides

    • Can detect antigens in solution or on cell membranes

  • T lymphocytes

    • Only recognize small peptides presented on MHC molecules

    • Require a specific environment (MHC) to trigger response

Major Histocompatibility Complex (MHC) Molecules

  • Two classes: Class I and Class II

  • Class I MHC

    • Expressed by all nucleated cells

    • Presents endogenous antigens (internal to the cell)

    • Activates CD8+ T Cytotoxic T Cells

  • Class II MHC

    • Expressed primarily by antigen presenting cells (APCs) like dendritic cells, macrophages, and B cells

    • Presents exogenous antigens (external pathogens)

    • Activates CD4+ Helper T Cells

Antigen Presentation Process

General Mechanism

  • Antigens are captured by dendritic cells or other APCs and processed internally

  • Antigen processing differs between Class I and Class II pathways

  • Importance of the MHC as a ‘silver plate’ for presenting peptide antigens to T cells

Dendritic Cells as Professional APCs

  • Activated upon antigen uptake and migration to lymph nodes

  • Upregulate surface molecules including MHC molecules and co-stimulatory signals (CD40, CD80, CD86)

  • Jargon: Dendritic cells are known for their function as professional APCs, crucial for naive T cell activation

Antigen Processing Pathways

Class I Pathway (Endogenous Antigens)

  • Intracellular antigens are broken down by proteasomes, producing peptides

  • Peptides enter the endoplasmic reticulum via TAP (Transporter associated with Antigen Processing)

  • Peptides are loaded onto MHC Class I molecules and transported to the cell surface

  • Activates CD8+ T cells, leading to targeted cytotoxic immune response

Class II Pathway (Exogenous Antigens)

  • Extracellular antigens are endocytosed and processed in lysosomes

  • Invariant chain helps stabilize Class II molecules until peptide loading

  • Results in CD4+ T cell activation, promoting help and response functions of other immune cells (e.g., B cells)

Key Concepts in Immune Response

  • MHC Restriction

    • T cells are restricted to recognize peptides only in the context of specific MHC molecules

    • Genetic diversity of MHC molecules allows for varied immune responses across individuals

  • Importance of lymphatic recirculation for T cell encounters with antigens

Summary of Learning Points

  • Antigen presenting cells (APCs) are essential for T cell activation

  • CD8 T cells recognize Class I MHC associated peptides; CD4 T cells recognize Class II

  • Hemoglobin expression reflects immune system diversity and adaptability

  • Future lectures will cover cross-presentation and variations in T cell activation

Conclusion

  • A specialized system is crucial to facilitate T cell encounters and responses to antigens

  • Importance of understanding antigen presentation as a key component in immunology

  • Encouragement to revisit key concepts and MHC properties in preparation for forthcoming topics.