Immunology: Humoral Immune Response and Allergy

Immunology Overview

Chapter 10: The Humoral Immune Response
  • B Cell Activation
    • By antigen and helper T cells
    • Thymus-dependent (TD) antigens: Require T cell help for B cell activation
    • Thymus-independent (TI) antigens: Can activate B cells without T cell help
    • Germinal Centers:
    • B cell proliferation and differentiation occur here
    • Somatic Hypermutation: Increases antibody affinity via point mutations in V regions
    • Isotype Switching: Changes antibody isotype (e.g., IgM to IgG, IgE)
    • Effector functions include transcytosis and tissue distribution of antibodies.
B Cell Development and Activation
  • Development:
    • B cells develop in the bone marrow and migrate to peripheral lymphoid organs
  • Activation:
    • Antigen Recognition: Naïve B cells recognize antigens presented by macrophages and follicular dendritic cells (FDCs)
    • Cross-Linking: Multiple B cell receptors (BCRs) binding to an antigen leads to activation.
Signals Required for B Cell Activation
  • Primary Signal: Recognition of antigens by multiple BCRs
  • Secondary Signal:
    • Involves CD40-CD40L interaction between T follicular helper (TFH) cells and B cells
    • Cytokine signaling from TFH cells promotes B cell survival and proliferation.
Germinal Center Dynamics
  • Germinal Center Formation:
    • Activated B cells proliferate and differentiate into
    • Plasma cells secreting antibodies or
    • Memory B cells
  • Zones:
    • Dark Zone: Proliferation and somatic hypermutation
    • Light Zone: Isotype switching and differentiation
  • Somatic Hypermutation Process:
    • Introduces mutations in V regions to improve affinity; requires activation-induced cytidine deaminase (AID)
Isotype Switching
  • Mechanism:
    • Changes in heavy-chain constant regions; occurs in the light zone of germinal centers
    • Can lead to production of various isotypes (IgG, IgE, IgA)
    • Essential signals provided by TFH cells.
Differentiation into Plasma and Memory Cells
  • Cytokine Influence:
  • TFH cell cytokines dictate whether B cells differentiate into plasma or memory cells.
Antibody Characteristics and Distribution
  • IgM: Largest isotype; predominant in blood
  • IgD: No major function; co-expressed with IgM
  • IgG: Most abundant in blood; crosses the placenta
  • IgE: Associated with mast cells; involved in allergic responses
  • IgA: Found in mucosal tissues; critical for secretory immunity
  • Transport Mechanisms:
    • Transcytosis of IgA involves poly-Ig receptor; FcRn transports IgG across endothelia.
Effector Functions of Antibodies
  • Mechanisms: Mediate protection through neutralization, opsonization, and complement activation.
    • Neutralization protects against viral infections by blocking toxin action.
  • Opsonization enhances phagocytosis of pathogens.
Chapter 14: Allergic Diseases and Hypersensitivity Reactions
  • Hypersensitivity Classifications:
    • Type I: Immediate; mediated by IgE; involves degranulation of mast cells
    • Type II: IgG-mediated; against new epitopes on cell surfaces, potentially leading to complement activation
    • Type III: Immune complex-mediated; small soluble complexes provoke inflammatory responses
    • Type IV: Delayed-type; T cell mediated responses to modified proteins.
Type I Hypersensitivity Reactions
  • Mechanism:
    • Involves interaction of allergens with IgE on mast cells, leading to release of inflammatory mediators (e.g., histamine)
  • Reactions: Include seasonal allergies, asthma, anaphylaxis, and food allergies.
Treatment Approaches for Allergies
  • Antihistamines: Block effects of histamine
  • Corticosteroids: General anti-inflammatory effects
  • Anti-IgE Therapy: Monoclonal antibodies can prevent IgE from activating mast cells.
Conclusion
  • Understanding the humoral immune response and the mechanisms of allergic reactions is crucial for developing therapeutic strategies against immune disorders and allergies.
Notes
  • Reinforced learning of key terms, mechanisms, and pathways of antigen presentation and antibody function is essential for a comprehensive understanding of immunology.