Humoral Immune Response Study Notes

ROSS UNIVERSITY

SCHOOL OF VETERINARY MEDICINE

Humoral Immune Response Study Notes

Presented by: George Nadăș, DVM, PhD, gnadas@usamvcluj.ro

Learning Objectives of Humoral Immune Response

  • Describe the activation of B lymphocytes and the role of co-stimulatory molecules.

  • Explain the necessary signals for B cell activation and their resulting consequences.

  • Describe B cells and how they respond to antigens.

  • Discuss the dynamics of antibody responses.

  • Detail the differentiation of B lymphocytes into plasma cells and memory cells.

  • Explain the structure of immunoglobulins.

  • Describe the effector functions of antibodies.

Overview of Humoral Immune Response

  • The humoral immune response is mediated by B cells, utilizing antibodies as effectors to neutralize and eliminate extracellular antigens.

  • Mature B cells express both IgM and IgD as receptors on their surface, critical for their function in the immune response.

Antibodies

  • Types of Antibodies:

    • IgA: Typically exists as a dimer and is about 32 nanometers in size when in this form.

    • IgM: Exists as a pentamer with a significant role in the immune response.

    • IgG: Monomeric form is about 8-10 nanometers.

Function and Timing of Antibodies

  • Antibody-mediated elimination occurs during:

    • Infections within mucosal organ lumen.

    • Defense mechanisms in fetuses and newborns.

  • Opsonization: Vitamin C and ascorbic acid assist in enhancing the opsonizing capability of antibodies.

  • Antibodies neutralize microbial toxins by blocking their binding to cellular receptors, rendering them ineffective upon attachment.

Targeting Microbes and Toxins

  • Antibodies can be effective against:

    • Intracellular and extracellular pathogens, with various mechanisms including opsonization and complement activation.

  • Example of Immune Mechanisms: Leukocyte

    • Fc Receptors: IgG mediated cytotoxicity through natural killer (NK) cells and macrophages (ADCC).

    • Eosinophils: Bind IgE for helminth destruction via granule release.

    • Complement Activation: Through the classical pathway.

B-Cell Antigen Receptor (BCR)

  • The BCR consists of approximately 200,000 to 500,000 soluble receptors per B cell, formed by:

    • Two heavy chains and two light chains associated with Igα (CD79a) and Igβ (CD79b).

Immunoglobulin Structure

  • Immunoglobulins contain:

    • 4 polypeptide chains: Two heavy chains and two light chains.

    • Constant and Variable Regions: Heavy chains have domains with both constant (CH) and variable (VH) components. Each chain typically consists of 400-500 amino acids.

  • Light Chains: May be kappa (κ) or lambda (λ) with corresponding variable (VL) and constant (CL) regions.

Antigen Binding Site

  • Composed of variable domains from both heavy and light chains linking through disulfide bonds.

  • Complementarity determining regions (CDRs): Key for binding to the specific epitope of an antigen, generally characterized by sequences of 6 to 10 amino acids.

B Cell Activation

  • Activation requires multiple signals including:

    • Antigen binding to BCR.

    • Co-stimulation by T helper cells, often through interactions with CD40, leading to further proliferation and differentiation into plasma cells.

  • Cytokines secreted by Th cells, such as IL-4, IL-5, IL-6, assist with class switching and enhancing the B cell response.

Mechanism of T-Dependent Antibody Response

  • T-dependent responses require helper T cells for:

    • Activation: Helper T cells stimulate B cells, leading to isotype switching and generation of high-affinity antibodies.

    • Affinity Maturation: A process leading to improved antibody binding through somatic mutations.

  • The generated antibodies include IgA, IgG, and IgE, each with distinct roles against various pathogens.

Differences Between T-Dependent and T-Independent Responses

  • T-Dependent: Involves protein antigens, T cell help, and high-affinity antibody production.

  • T-Independent: Involves non-protein antigens (e.g., polysaccharides), often resulting in quick low-affinity IgM responses without T cell assistance.

Memory B Cells

  • Generated primarily in T-dependent responses, capable of rapid subsequent responses to reintroduced antigens.

  • Memory cells express high levels of BCL-2, allowing their longevity even without continuous antigen exposure.

Antibody Feedback Mechanism

  • Secreted antibodies regulate their own production through feedback inhibition.

    • Mechanism: Antigen-antibody complexes can bind to Fc receptors on B cells, triggering inhibitory signals to downregulate further antibody production.

Conclusion

  • The study of humoral immunity emphasizes the roles of various B cell subtypes, the processes involved in antibody diversity, and the significance of T-cell interactions in shaping robust adaptive immune responses.