immune_2

Page 1: Introduction to Antibodies

• Introduced by Associate Professor Tatyana Chtanova from UNSW Sydney.

• Image sourced from CDC.gov.

Page 2: Learning Outcomes

• Describe antibody structure: Understanding the physical and chemical makeup of antibodies.

• Outline antigen binding: Explanation of how antibodies interact with antigens.

• Differentiate Affinity vs Avidity: Affinity refers to the strength of a single interaction, while avidity refers to the total strength from multiple interactions.

• Describe antibody classes: Overview of the different types of antibodies and their functions.

• Outline antibody functions: Explanation of the roles antibodies play in the immune system.

• Discuss maternal IgG: Insights into the transfer of antibodies from mother to child.

• Explain principles of vaccination: Understanding how vaccines work in inducing immunity.

Page 3: Case Report

• Patient: 7-month-old baby boy.

• Symptoms: Frequent ear, throat, lung, and sinus infections.

• History: Persistent severe infections and very small tonsils.

• Blood Test Findings: Extremely low numbers of B cells and very low levels of immunoglobulins (antibodies).

Page 4: Functions of Antibodies

• Protection from infection through various mechanisms:

  • Neutralization of pathogen infectivity.

  • Enhancing phagocytosis.

  • Antibody-dependent cellular cytotoxicity (ADCC).

  • Activation of immune cells.

  • Complement-mediated lysis of pathogens or infected cells.

Page 5: Locations of Antibodies

• Present in:

  • Plasma/Serum: Comprises ~20% of protein in blood plasma.

  • Secretions: Found in milk, saliva, tears, and intestinal fluid.

  • Cell Surfaces: Integral to B cells and receptors on NK cells, macrophages, and granulocytes.

Page 6: Immunoglobulin Structure

• Molecular Weight: Polypeptide approximately 150-180 kDa.

  (Molecular Weight of Immunoglobulin: Polypeptide approximately 150-180 kDa.)

• Composition: 2 identical light chains (L) and heavy chains (H) linked by disulfide bonds.

• Each chain consists of variable (V) and constant (C) regions:

  • L Chain: ~25 kDa, consists of 1 V and 1 C region.

  • H Chain: ~50 kDa, contains 1 V and 3-4 C regions.

    ### Variable and Constant Regions in Antibodies - **Variable Regions (V):** - Responsible for binding to specific antigens. - Create the antigen-binding site and determine antibody specificity. - **Constant Regions (C):** - Dictate the antibody's class and function in the immune response. - In heavy chains, consist of 3-4 C regions (~50 kDa), while light chains have 1 C region.

Page 7: Antigen Binding in Immunoglobulins

• The V regions of light and heavy chains create an antigen-binding site.

• Each immunoglobulin has two antigen binding sites.

• Affinity: Strength of interaction between one antigen-binding site and its antigen.

• Avidity: Total strength from multiple binding sites.

Page 8: Types of Light and Heavy Chains

• Light Chains: Two types (kappa and lambda), each ~25 kDa.

• Heavy Chains: Five types, ranging from 50-70 kDa, differing in size and composition.

  ### Functions of Light Chain Types - **Kappa Chains:** An important component of antibodies, kappa chains can combine with any heavy chain to form a functional immunoglobulin. - **Lambda Chains:** Similar in function to kappa chains, lambda chains also contribute to the formation of antibodies. ### Types of Heavy Chains and Their Functions 1. **IgM Heavy Chain:** Forms a pentamer; effective in activating the complement system during the primary immune response. 2. **IgD Heavy Chain:** Primarily found on the surface of naïve B cells; its specific function remains uncertain. 3. **IgG Heavy Chain:** The most abundant antibody in serum; provides long-term immunity and can cross the placenta to protect the fetus. 4. **IgA Heavy Chain:** Predominantly found in mucosal secretions; protects mucosal surfaces and is critical in milk, saliva, and tears. 5. **IgE Heavy Chain:** Functions in defense against parasitic infections and mediates allergic reactions; involved in anaphylaxis.

Page 9: Immunoglobulin Genetics

• Coded regions:

Constant Region

• Definition: The constant region of an antibody determines its class (isotype) and function.

• Isotypes: Different types of antibodies (e.g., IgM, IgG) have distinct constant regions which confer specific functional roles in the immune response. For example:

  • IgM: effective in activating the complement system during primary immune response.

  • IgG: provides long-term immunity and can cross the placenta.

Variable Region

• Definition: The variable region is responsible for binding to specific antigens and creating the antigen-binding site.

• Genetic Coding: The variable region is coded by 3 separate gene segments: V (variable), D (diversity), and J (joining). This configuration allows for a vast diversity of antibodies, enabling the immune system to recognize a wide array of antigens with high specificity.

• Functionality: The unique combination of these gene segments contributes to the specificity of the antibody, enabling it to bind efficiently to its corresponding antigen.

• Different isotypes have distinct constant regions (e.g., IgM, IgG).

• Variable region coded by V, D, and J segments, conferring specificity.

Page 10: Immunogenetics

• Each B cell has unique specificity; requires distinct genes for unique proteins.

• Humans possess about 20,000 genes capable of producing millions of different antibodies through complex genetic mechanisms.

  Genetic mechanisms for antibody diversity include: 1. **Combinatorial Diversity:** This mechanism involves the combination of different gene segments (V, D, J) during B cell development, resulting in a wide variety of possible antibodies. For heavy chains, there are approximately 38-46 V genes, 25 D genes, and 6 J genes, contributing to millions of antibody combinations. 2. **Somatic Hypermutation:** This process occurs after a B cell has encountered an antigen. It introduces mutations into the variable region of the antibody genes at a high rate, allowing for the selection of B cells that produce antibodies with higher affinity for the antigen. 3. **Class Switching:** After activation, B cells can switch from producing one class of antibody (e.g., IgM) to another (e.g., IgG, IgA) without altering the specificity for the antigen. This is influenced by cytokines and is crucial for adaptive immune responses.

Page 11: B Cell Development

• B cells migrate to the bone marrow, mature, and then enter the bloodstream for further differentiation.

  B Cell Development Process: - **Migration to Bone Marrow**: B cells migrate to the bone marrow where they begin their development. - **Maturation**: In the bone marrow, B cells undergo maturation, which involves rearranging their immunoglobulin genes to produce unique antibodies. - **Exit to Bloodstream**: Once mature, B cells leave the bone marrow and enter the bloodstream for further differentiation and activation in response to foreign antigens.

Page 12: Immunoglobulin Gene Construction

• Functional antibody genes are formed by joining selected V, D, and J segments.

• Rearrangement occurs in heavy chains (VDJ) and light chains (VJ).

  Rearrangement in antibodies is a critical process that occurs during B cell development. In heavy chains, the rearrangement involves V (variable), D (diversity), and J (joining) gene segments, often referred to as VDJ recombination. This allows for a vast diversity of antibodies because it combines different gene segments to produce unique heavy chain variants. In contrast, light chains undergo a simpler rearrangement process involving only V and J segments, called VJ recombination. This arrangement enables B cells to produce a wide array of antibody specificities, essential for recognizing diverse antigens.

Page 13: Genetic Diversity in Antibodies

• Combinatorial Diversity: Results in a vast range of antibody specificities:

  • Heavy chains: 38-46 V genes, 25 D genes, 6 J genes.

  • Kappa chains: 40 V genes, 5 J genes.

  • Lambda chains: 30 V genes, 4 J genes.

• Resulting in approximately 2,160,000 combinations.

Page 14: Clonal Selection Theory

• Developed by Macfarlane Burnet, based on earlier work by immunologists.

• Each B cell has a unique specificity and can recognize only one antigen.

• Naïve B cells circulate until they meet an antigen, mainly occurring in secondary lymphoid organs.

Page 15: Activation Following Antigen Encounter

• Antigens bind to specific B cells, leading to their activation.

• Activated B cells undergo clonal expansion: rapid division and differentiation into plasma or memory B cells.

  Activated B cells undergo clonal expansion following antigen encounter, which involves rapid division and differentiation into plasma cells and memory cells. - **Plasma Cells:** These are the effector B cells responsible for producing and secreting large amounts of antibodies to help eliminate the antigen. Plasma cells are short-lived and immediately respond to the detected pathogens. - **Memory Cells:** These B cells are long-lived and remain in the body after the initial immune response has concluded. They are crucial for providing faster and more robust immune responses upon subsequent exposures to the same antigen, allowing for a quicker and more effective immune reaction.

Page 16: Constant Region Functions

• Fc functions: Mediate various immune responses:

  • Activate the complement system via antigen-antibody complexes.

  • Phagocyte Fc receptors enable ingestion of pathogens.

    • Many cells express Fc receptors because these receptors help them recognize and bind to antibodies that are attached to pathogens (like bacteria and viruses). When a pathogen is marked by antibodies, phagocytes and other immune cells use Fc receptors to grab onto these antibody-pathogen complexes. This helps them ingest and destroy the pathogens more effectively, playing a crucial role in the body’s immune response to keep us healthy.

Page 17: Antibody Classes

• Five primary classes:

  • IgM, IgD, IgG, IgA, and IgE, characterized by their constant regions.

  • IgG: has four subclasses; IgA: has two subclasses.

Page 18: Structural Variants of Antibodies

• Monomer: IgD, IgE, IgG.

• Dimer: IgA.

• Pentamer: IgM.

Page 19: IgM Characteristics

Difference between Monomeric IgM and Secreted Pentameric IgM:

• Monomeric IgM:

  • Found on the surface of B cells.

  • Exists as a single unit (monomer).

  • Functions as a B cell receptor, crucial for antigen recognition.

• Secreted Pentameric IgM:

  • Found in serum after activation of B cells.

  • Exists as a five-unit structure (pentamer).

  • Highly effective in activating the complement system during the primary immune response.

• Highly effective in activating complement despite low affinity and primarily produced during primary immune response.

• IgM's high effectiveness in activating the complement system is primarily due to its pentameric structure when secreted, allowing it to bind multiple antigens simultaneously. This multivalency enhances its ability to activate complement pathways, leading to pathogen opsonization and lysis, which are critical during the initial immune response to infections.

Page 20: IgG Characteristics

• Monomeric with four subclasses. IgG has four subclasses: IgG1, IgG2, IgG3, and IgG4. Each subclass differs in their hinge regions and complement activation capabilities. - **IgG1:** Most abundant; provides strong opsonization and neutralization. - **IgG2:** Responds primarily to polysaccharide antigens; less effective at activating complement. - **IgG3:** Highly effective in activating complement and provides strong immune response. - **IgG4:** Anti-inflammatory; involved in responses to allergens.

• IgG antibodies have the longest serum half-life (21-30 days) and are crucial in the secondary immune response due to their ability to provide long-term immunity. Upon re-exposure to the same antigen, memory B cells rapidly produce IgG, leading to a faster and more robust immune response, thereby enhancing the body's ability to fight off the pathogen effectively.

• Opsonizes pathogens and crosses the placenta.

• Activates complement pathways, which helps to enhance phagocytosis and recruit additional immune cells to the site of infection.

Page 21: Ig Levels Post-Birth

• Maternal IgG transferred passively contributing to neonatal immunity.

• IgG levels fluctuate during infancy, with measurable increases as the child grows.

Page 22: IgG Subclasses and Functions

• IgG subclasses (IgG1, IgG2, IgG3, IgG4) differ in their hinge regions, which affect their flexibility and antigen binding. Additionally, they vary in their abilities to activate the complement system, with IgG3 being the most effective, followed by IgG1. IgG2 has a reduced capacity for complement activation, while IgG4 does not activate complement effectively at all.

Page 23: IgA Characteristics

• Predominant in mucosal secretions and major antibody in milk, saliva, and tears.

• Exists as monomers in serum and dimers (secretory IgA) on mucosal surfaces.

• Monomers: Single units of antibodies found in serum. For example, IgD, IgE, and IgG antibodies exist as monomers.

• Dimers: Two units of antibodies linked together. Secretory IgA is a well-known example that exists as a dimer on mucosal surfaces, providing enhanced protection against pathogens.

Page 24: Secretory IgA Function

• slgA consist of 2 IgA units joined by a J chain

• Secretory IgA (sIgA) prevents microbial attachment and inhibits pathogen colonization in mucosal areas.

• More resistant to proteolytic enzymes compared to IgA1.
  

  **J Chain:** - A polypeptide that is important for the polymerization of certain immunoglobulins, particularly associated with secretory IgA and IgM. - It facilitates the formation of dimeric IgA and pentameric IgM by linking their units together, which enhances their functionality. **Isotypes of IgA in Humans:** - Humans have two isotypes of IgA: 1. **IgA1:** Predominantly found in serum and is a major isotype in mucosal secretions. 2. **IgA2:** Found mainly in mucosal secretions and is more prevalent in secretions than in serum. **Functions of Secretory IgA (sIgA):** - Prevents microbial attachment and inhibits pathogen colonization in mucosal areas. - More resistant to proteolytic enzymes compared to IgA1, ensuring its function in hostile environments like the gut and respiratory tract.

Page 25: IgD Characteristics

• Monomeric, detected at very low levels in serum, with uncertain function.

• Found on naïve B cells alongside IgM.

Page 26: IgE Characteristics

• Produced in minimal quantities but plays a significant role in defense against parasites and allergic reactions.

• Responsible for anaphylactic responses upon receptor binding.

  ### Functions of IgE - **Defense Against Parasites:** IgE is primarily involved in the immune response against parasitic infections, helping to combat these pathogens effectively. - **Allergic Reactions:** IgE plays a critical role in mediating allergic reactions, such as hay fever, asthma, and food allergies. - **Anaphylactic Response:** When IgE binds to receptors on mast cells and basophils, it can trigger severe allergic reactions, known as anaphylaxis, releasing histamines and other mediators that lead to inflammation and other symptoms.

Page 27: Summary of Immunoglobulin Classes

• Comparison of Ig classes based on their structure, function, and serum presence, including half-life and ability to cross the placenta.

• IgG: 80% of serum, long-term immunity.

• IgA: Secretory antibody, essential at mucosal surfaces.

• IgM: Initial response antibody, multimeric.

• IgD: Monomeric, detected at very low levels in serum with uncertain functions and found on naive B cells alongside IgM

• IgE: Allergy and parasitic defense.

Page 28: Class Switching Mechanism

• Naïve B cells express IgM and IgD.

• Upon activation, they can undergo class switching influenced by T cell help, resulting in diverse antibody isotypes while maintaining specificity.

• The Constant region of the antibody changes during class switching; the variable regions, and therefore antigen specificity, remain unchanged.

  This process allows for the production of antibodies such as IgG, IgA, and IgE, each tailored for different functions in the immune response.

  • Class switching is like changing your clothes for different occasions. When B cells (a type of immune cell) first react to a germ, they make a certain type of antibody called IgM, which is like wearing a basic outfit. But later, if they need to fight that germ more effectively or deal with different germs, they can "change" to other types of antibodies, like IgG or IgA, which help them work better in those situations. So, it's all about using the right tools for the job while still targeting the same germ.

During class switching, B cells change the constant region of an antibody so they can make different types of antibodies (like IgM or IgG). This is important because each type of antibody has a special role in fighting infections. However, the part of the antibody that helps it recognize a specific germ (the variable region) stays the same. This way, the B cell can still target the same germ, but now it can use different tools to help get rid of it better

Page 29: Measuring Antibodies

• Antibody presence indicates prior infection;

• IgM levels suggest recent infection due to their shorter lifespan compared to IgG.

Page 30: Utility of Antibodies

• Prevent pathogen attachment, enhance phagocytosis, activate complement system, and initiate ADCC.

Page 31: Antibody Response Timing

• Primary Response: initial antigen exposure leads to slower antibody production (IgM initially).

• Secondary Response: faster and more robust due to memory cells (predominantly IgG).

Page 32: Vaccination and Antibodies

• Vaccines induce immunological memory, leading to a potent secondary response upon actual infection.