BIOL 3120 LECTURE 7

Course Overview

Course Information

  • Course Name: Biol3120 Immunobiology Lecture 7: Antibody Structure

  • Instructor: Yuqing Feng, Ph.D

  • Date: January 27, 2025

  • Chapters Covered:

    • Chapter 4, pages 97-107; pages 118-123

    • Chapter 9, pages 260-265

Term Test Information

Upcoming Term Test 1

  • Date: Wednesday, January 29

  • Location: ACW 206 and ACW 205

  • Time: 10 AM

  • Requirements: Bring student ID for entry

  • Content Focus: The test will include material covered in slides and lecture notes, assessing understanding of key concepts discussed in class.

  • Note: No inquisitive materials, such as textbooks or notes, are allowed during the test to ensure academic integrity.

Learning Objectives

  1. Identify antibody regions: Understand the specific regions of antibodies that contribute to their ability to recognize and bind to specific antigens, interacting with various immune components.

  2. Summarize antibody structure: Describe the structural organization of the antibody’s antigen-binding site and how it interacts with antigenic epitopes, highlighting the significance of these interactions in immune response.

  3. Explain monoclonal antibodies: Detail the engineering process of mouse monoclonal antibodies, including their development to mimic human antibodies and explore their applications in therapeutic settings.

  4. Elucidate effector functions: Describe the five classes of antibodies, their distinct effector functions, and their anatomical roles within the immune system.

Antigen Receptors for B Cells

Immunoglobulin (Ig)

  • Two Forms:

    • Membrane-bound form: Known as the B cell receptor (BCR), which is integral for the initial recognition of antigens by B cells.

    • Secreted form: Commonly known as antibodies, which play a critical role in the immune defense by neutralizing pathogens.

  • Specificity: Both BCRs and antibodies exhibit the same antigen specificity, which is crucial in ensuring an effective adaptive immune response.

Pattern Recognition

  • Mechanism: Innate immune cells, such as macrophages and neutrophils, utilize pattern recognition receptors to identify foreign antigens quickly.

  • B Cell Activation: Activated B cell receptors recognize bacterial membrane epitopes, which leads to the differentiation of B cells into antibody-secreting plasma cells, thereby amplifying the immune response.

Plasma Cell Functionality

Plasma Cells

  • Specialization: Plasma cells are highly specialized B cells designed primarily for the production of antibodies.

  • Structural Adaptations: These cells possess extensive rough endoplasmic reticulum (RER), enabling them to synthesize large quantities of antibodies efficiently, which are then secreted to combat infections.

Antibody Structure

Basic Structure

  • Composition: Antibodies are composed of four polypeptide chains, specifically:

    • 2 Heavy Chains: Each approximately 50 kDa in size.

    • 2 Light Chains: Each approximately 25 kDa in size.

  • Connection: The chains are linked together via disulfide bonds, creating a stable structure.

  • Shape: Antibodies typically adopt a Y-shaped structure, with the aminoterminal (N-terminus) at the tips of the Y and the carboxy terminal (C-terminus) at the base.

Regions of the Antibody

  • Variable and Constant Regions:

    • The heavy and light chains comprise constant (C) and variable (V) regions.

    • Variable Domains: Identified as VH (heavy) and VL (light), play a crucial role in antigen binding.

    • Constant Domains: Comprise CH (heavy chain) and CL (light chain), which maintain structural integrity and mediate immune interactions.

  • Binding Sites: The antigen-binding sites are formed by the combination of the VH and VL domains, which facilitate specific interactions with antigens.

Immunoglobulin Folding

Protein Domains

  • Folding Mechanism: The immunoglobulin chains undergo folding into stable domains, creating specific structural features that are essential for their immune functions.

Antigen-Binding Sites

Structure and Function

  • Binding Site Configuration: Each antibody contains two identical antigen-binding sites, resulting from the combination of variable regions.

  • Domain Composition: Each light chain consists of 2 domains, while each heavy chain contains 4 domains.

  • Functional Importance: The antigen-binding sites are crucial for facilitating specific interactions with various antigens, thus impacting the immune response.

Key Points on Antibody Structure

  • Composition Summary: Comprises 2 heavy chains (with 4 Ig domains each) and 2 light chains (with 2 Ig domains each), all stabilized by disulfide bonds.

  • Functional Regions Importance: The variable regions determine antigen specificity, while the constant regions mediate various immune interactions.

  • Flexibility Enhancements: The hinge region between the heavy chains adds flexibility, essential for optimal antigen binding and enhancing the antibody's ability to engage with antigens effectively.

Digestion of Antibodies by Papain

Enzyme Function

  • Cleavage Mechanism: The enzyme papain cleaves IgG antibodies above the hinge region, generating:

    • 2 Fab Fragments: These contribute to antigen binding but lack effector functions.

    • 1 Fc Fragment: Essential for mediating immune functions.

  • Significance: This separation between binding (Fab) and immune response (Fc) is pivotal for diagnostic and therapeutic applications.

Papain and Immunology

Key Details on Papain

  • Source: Papain is derived from papaya and is not synthesized in the human body.

  • Research Applications: It is extensively utilized in biological research and various applications due to its unique enzymatic properties.

Immunoglobulin Domain Structure

Domain Composition

  • Structural Features: Ig domains are characterized by two layers of beta-sheets, which are reinforced through hydrophobic interactions, crucial for stability and functionality.

Key Points on Ig Domain Structure

  • Stabilization Mechanisms: Disulfide bonds and hydrophobic interactions contribute to the stability and functionality of Ig domains, ensuring proper structural integrity.

  • Fragment Structure: The Fab fragment consists of one light chain domain and a part of the heavy chain, central to its antibody activity.

Ig Superfamily Proteins

Common Structure

  • Shared Domain: Proteins within the Ig superfamily share a common Ig domain structure, which is fundamental in various biological functions.

  • Examples: The Ig superfamily encompasses immunoglobulins, T-cell receptors, MHC proteins, and adhesion molecules that play pivotal roles in immune signaling and recognition.

Key Points on Ig Superfamily Proteins

  • Functional Roles: These proteins are significantly involved in immune signaling, antigen recognition, and cell adhesion processes, critical for effective immune responses.

Hinge Region Flexibility

Hinge Importance

  • Functionality: The hinge region facilitates antibody flexibility, allowing it to bind to multiple conformations of antigens.

  • Benefits: Enhanced flexibility increases the efficiency of binding to a variety of pathogens, thus improving immune response capabilities.

Key Points on Hinge Region and Flexibility

  • Enhanced Binding Effectiveness: Flexibility provided by the hinge region allows antibodies to engage effectively with different antigens, maximizing their functional potential.

Hypervariable Regions of Antibodies

Contributions to Antigen Binding

  • Specificity Role: Hypervariable regions (HV) are crucial determinants of antigen specificity.

  • Framework Regions (FR): These regions provide vital structural support for the HV regions, ensuring proper folding and functionality of the antigen-binding site.

Key Points on Antibody Variable Regions

  • Functionality of HV and FR: The hypervariable regions are directly involved in binding, while framework regions provide the essential structure necessary for effective antigen recognition.

CDRs in Antibody Diversity

Location and Function

  • Contribution to Diversity: Complementarity-determining regions (CDRs) are integral in forming the antigen-binding site and play a pivotal role in the recognition of unique antigens, adding to the diversity of the antibody repertoire.

Page Summary on Antibody Binding

Complementary Binding

  • Binding Mechanism: The structural composition of antibodies is specifically designed to complement the characteristics of antigens, allowing for effective and efficient binding, which is essential for the immune response.

Key Points on Antibody-Antigen Binding Specificity

Mechanism Overview

  • Binding Criteria: The binding mechanism through CDRs ensures specificity based on the size and shape of the antigen, which is critical for the precision of immune recognition and response.

Epitopes and Antibody Recognition

Types of Epitopes

  • Classification: Epitopes can be categorized into linear (continuous) and discontinuous (conformational) types. Antibodies recognize these based on their structural forms, allowing for precise immune targeting.

Key Points on Epitopes

  • Definitions:

    • Linear Epitopes: Recognized in both native and denatured states.

    • Discontinuous Epitopes: Only recognized in their native shapes, highlighting the importance of protein conformation in immune recognition.

Multivalent Antigens

Definition

  • Characteristics: Multivalent antigens possess multiple epitopes that can bind to various antibodies, allowing for a more robust and varied immune response, enhancing the effectiveness of the body's defense mechanisms.

Key Details on Antigen-Antibody Interactions

Binding Strength

  • Correlation: The strength of antigen-antibody interactions correlates directly with the fit between the surfaces of the antibody and antigen, emphasizing the importance of structural compatibility in immune responses.

Key Concepts in Passive Immunization

Role of Emil Von Behring

  • Historical Significance: Emil Von Behring pioneered serum therapy, utilizing antibodies obtained from immune individuals to provide protection against diseases, marking a significant advancement in immunology and therapeutic interventions.

Serum Therapy for Diphtheria and Tetanus

Historical Significance

  • First Documented Use: This therapeutic approach was the first documented method used to convey immunity against bacterial toxins through antibody transfer, representing a milestone in medical history and immunotherapy.

Monoclonal Antibody Production Overview

Definition

  • Characteristics: Monoclonal antibodies are produced from identical clones of B cells, allowing for the targeting of specific antigens in a uniform manner, widely utilized in diagnostics and therapies.

Key Concepts in Hybridoma Technology

Monoclonal Antibody Basics

  • Production Method: Monoclonal antibodies are formed from a single B cell clone that is fused with myeloma cells, creating hybridomas that can proliferate continuously, leading to sustained production of specific antibodies.

Production Steps

  • Details: The hybridomas are cultivated and selected for their ability to produce the desired antibodies, which are then harvested and purified for use.

Flow Cytometry and Applications

Technique Overview

  • Description: Flow cytometry is a powerful technique that utilizes fluorescently labeled antibodies to identify and analyze specific cell surface markers, providing valuable information about cellular characteristics and functions.

Applications

  • Utilization: Flow cytometry is extensively used in clinical diagnostics, cancer research, and monitoring immune responses, making it a critical tool in immunological studies and therapeutic applications.

Monoclonal Antibody Treatments

Versatility in Use

  • Clinical Applications: Monoclonal antibodies are highly versatile and play a crucial role in cancer therapies and treatments for autoimmune diseases, highlighting their importance in modern medicine.

IgG Structure and Functionality

Diversity of IgG

  • Subclasses: Different subclasses of IgG have distinct structural features that influence their binding properties and functional performance, underscoring the adaptability of the immune response to various challenges.