3.5

Lecture Overview

• Lecturer: Tim Maley, PhD
• Course: PSIO 202, University of Arizona
• Contact Information:
• Email: maleytim@arizona.edu
• Office: Gittings 113
• Office Hours: T/R 10-11am

Learning Objectives (Saladin, Ch. 21)

• Diagram and explain the steps involved in an antibody-mediated immune response.
• Explain how antibodies initiate destruction of infected cells and antigens.
• Compare and contrast the different classes of antibodies and their functions.
• Explain the basis for immunological memory and how it enables long-term protection.
• Compare and contrast cell-mediated and antibody-mediated immune responses.

The Immune Response

• Humoral immunity (antibody-mediated immunity):
• Utilizes antibodies to tag disease agents.
• Indirect method of attack; antibodies facilitate the destruction rather than actively attacking pathogens.
• Establishes a memory of past infections, aiding future immunity.

Steps in Antibody-Mediated Immunity

1. Antigen Presentation: Antigens are presented to T cells by antigen-presenting cells (APCs).
2. Antigen Recognition: Recognition occurs when B cells and T helper cells bind to specific antigens.
3. Activation: B cells and T cells are activated upon recognition, often requiring a helper T cell.
4. Proliferation and Differentiation: Activated B cells proliferate and differentiate into plasma cells and memory B cells.
5. Destruction/Inactivation: Antibodies act to neutralize or destroy extracellular antigens.

B Cell Antigen Receptors

• B cells have receptors that can bind to various antigens, and when activated, they produce antibodies (soluble receptors).
• Antibody structure includes:
• Two parallel protein chains (light and heavy chains).
• Constant region determines antibody class; variable region binds specific antigens.

Activation Process Overview

• B Cell Activation:
• Inactive B cells bind free antigens.
• Bound antigens undergo endocytosis and presentation on MHC II molecules.
• Helper T cells provide secondary signaling (costimulation) to activate B cells.
• Clonal Expansion:
• Activated B cells proliferate to form clones, which can differentiate into:
  • Plasma Cells: Secrete antibodies (short-lived).
  • Memory B Cells: Long-lived, provide rapid response upon re-exposure.

Mechanisms of Antibody Action

• Defensive Mechanisms Used by Antibodies:

1. Neutralization: Blocking pathogen binding.
2. Immobilization: Preventing movement of pathogen.
3. Complement Activation: Triggering a cascade that opsonizes pathogens.
4. Enhancement of Phagocytosis: Marked pathogens are easier targets for phagocytes.
5. Agglutination & Precipitation: Clumping agents for removal.
6. Enhancing Inflammation: Recruiting immune cells to the site.

Classes of Antibodies

1. IgD: Role in B cell activation; serves as an antigen receptor on B cells.
2. IgM: First antibody produced; excellent at complement activation and neutralizing viruses.
3. IgA: Predominant in mucosal areas; protects surfaces like gut and respiratory tract.
4. IgG: Most abundant in serum; provides the majority of antibody-based immunity against invading pathogens.
5. IgE: Binds to allergens and triggers allergic reactions; important in defense against parasites.

Immunological Memory

• Primary Response: Antibody concentration peaks approximately 7-10 days after first exposure to an antigen.
• Secondary Response: Memory B cells respond more rapidly and robustly, resulting in an exponential increase in antibody titers.

Types of Acquired Immunity

• Naturally Acquired: Through infection or maternal antibodies.
• Artificially Acquired: Via vaccination or introduced immune serum.
• Active Immunity: Body produces its own antibodies.
• Passive Immunity: Antibodies provided from another source.

Comparison of Cellular and Humoral Immunity

Summary of Immune Response

• Cellular Immunity Pathway: Involves T cells directly attacking infected cells.
• Humoral Immunity Pathway: Involves B cells producing antibodies that target pathogens in bodily fluids.
• Both involve intricate signals and interactions among various immune cells to ensure effective responses against pathogens.