Ags Ag Receptors Lecture C BMS2045 2025

Antigens and Antigen Receptors

Overview

Lecture by: Dr. Natalie Riddell (n.riddell@surrey.ac.uk)Theme: Discussion Board for 12AY02

Theme 3 Lecture Content

Lecture Schedule

• Lecture A (Today):

  • Part 1: General Principles, Antigens, and Immunogens

  • Part 2: The B Cell Receptor and Antibodies

• Lecture B (Wednesday, 10 AM):

  • Part 1: Epitopes, Affinity, and Avidity

  • Part 2: The T Cell Receptor, Polyclonal Responses

• Lecture C (Thursday, 2 PM):

  • Part 1: MHC Class I & II, Antigen Processing and Presentation

  • Part 2: Genetics of Antigen Recognition

Lecture C Content

Key Topics

• Structure and Function of MHC Class I and II

• Dendritic Cells

• Mechanisms of Antigen Processing and Presentation

Detailed Structure of MHC Class I and II

Learning Objectives

• Illustrate the structure of MHC Class I, including its specific domains and their roles in antigen binding.

• Describe the MHC Class I antigen presentation process, including the steps involved in antigen processing and the significance of cytosolic pathways.

• Illustrate the structure of MHC Class II, highlighting the dual-chain composition and its implications for antigen presentation.

• Describe the MHC Class II antigen presentation process, focusing on how exogenous antigens are processed and presented to T cells.

• Compare antigen presentation mechanisms between MHC Class I and II, emphasizing differences in the nature of antigens they present and the type of T cells they interact with.

• List different Antigen Presenting Cells (APCs) such as dendritic cells, macrophages, and B cells, and discuss their roles in immunity.

• Explain the critical role of Dendritic Cells (DCs) in antigen capture, processing, and presentation to T cells, which is vital for initiating adaptive immune responses.

Antigen Receptors

• B Cells and T Cells:

  • Paratopes: Antigen binding sites located on B Cell Receptors (BCRs) and T Cell Receptors (TCRs) that specifically interact with epitopes.

  • Epitopes: Specific regions or structures on antigens that are recognized by receptors, dictating the immune response.

  • Overview of the essential roles played by MHC Class I and Class II molecules in T cell activation and interactions; MHC Class I interacts primarily with CD8 T cells, while MHC Class II interacts with CD4 T cells.

Mechanisms of Antigen Presentation

T Cell Processing

• T cells primarily interact with linear peptide arrays consisting of approximately 9 amino acids.

• Antigens undergo processing to reveal their primary amino acid structure for recognition.

• Antigens presented by APCs such as macrophages and dendritic cells are crucial for T cell activation.

Classification of Antigens

• Endogenous Antigens: Presented by MHC Class I molecules to CD8 T cells (Cytotoxic T cells) and derived from intracellular sources.

• Exogenous Antigens: Presented by MHC Class II molecules to CD4 T cells (Helper T cells) and typically originate from external sources.

MHC Class I and II Structure

• MHC Class I:

  • Present on all nucleated cells and platelets, playing a pivotal role in immune surveillance.

  • Composed of a single polypeptide chain associated with β2-microglobulin; forms a peptide-binding groove mainly from α1 and α2 domains for antigen binding.

• MHC Class II:

  • Composed of two chains (α and β), and primarily found on professional APCs (Dendritic Cells, Macrophages, B Cells).

  • Both chain domains contribute significantly to the formation of the peptide binding site.

Features of Peptides Binding to MHC

• MHC Class I vs Class II:

  • Binding Specificity: MHC Class I binds peptide fragments of approximately 8-10 amino acids in length, whereas Class II binds longer fragments, around 13-18 amino acids.

  • Each MHC molecule displays one peptide at a time for T cell recognition.

  • MHC molecules require bound peptide for stable expression on the cell surface, which is critical for T cell education and activation.

Dendritic Cells (DCs)

Characteristics and Function

• Location: Dendritic cells reside in various tissues including the skin, gastrointestinal tract, and respiratory system, adapting their function based on location.

• Immature DCs (Langerhans cells) are primarily located in the epidermis and act as sentinels for pathogen detection.

• Upon activation, Mature DCs migrate to lymph nodes where they effectively capture, process, and present antigens to na\