UNIT 6: DEFENSE & IMMUNITY

Immune System Overview

Types of Immune Responses:

• Non-specific (innate):

  • Do not respond to specific antigens.

  • Provide immediate, general defense against pathogens.

  • Includes physical barriers (skin), phagocytes (macrophages, neutrophils), and chemical mediators (complement proteins, cytokines).

• Specific (adaptive):

  • Respond to specific antigens with tailored responses.

  • Involves the activation of lymphocytes (T-cells and B-cells) and the production of antibodies.

Cells Mediating Immune Responses and Their Functions:

• Macrophages: Target and phagocytose dead/dying cells and pathogens, present antigens to T-cells.

• Eosinophils: Fight parasitic infections and mediate allergic responses.

• Natural Killer (NK) Cells: Identify and destroy abnormal or infected cells without needing prior sensitization.

• T-cells:

  • Cytotoxic T-cells (Tc): Kill infected or cancerous cells.

  • Helper T-cells (Th): Activate B-cells and macrophages and enhance the immune response.

• B-cells: Produce antibodies in response to specific antigens and can function as antigen-presenting cells (APCs).

Chemical Factors Influencing the Immune System:

• Cytokines:

  • Signaling proteins that mediate and regulate immunity, inflammation, and hematopoiesis.

  • Types include interleukins, interferons, and tumor necrosis factor (TNF).

• Complement Proteins:

  • Involved in opsonization (marking pathogens for destruction), promoting inflammation, and forming the membrane attack complex (MAC) that disrupts pathogen membranes.

  • Activated via classical, alternative, or lectin pathways.

Complement System

• Classical Pathway: Initiated by antibody-antigen complexes.

• Alternative Pathway: Activated directly on microbial surfaces.

• Lectin Pathway: Activated by mannose-binding lectin (MBL), which binds to carbohydrates on pathogens.

• Common Pathway: Involves cell lysis through the formation of MAC that leads to pathogen destruction and inflammation.

Innate vs. Acquired Immunity:

• Innate Immunity:

  • Present at birth, involves physical barriers and immune cells that respond immediately to infections without previous exposure.

• Acquired Immunity:

  • Develops after exposure to specific antigens, involves memory cells (B-cells and T-cells).

  • Can be induced (e.g., vaccination) or natural (e.g., infection).

Comparing Specific and Non-Specific Defenses:

• Specific Defenses:

  • Target specific pathogens, involve slow activation, and provide immunological memory.

• Non-Specific Defenses:

  • Rapid response to all pathogens, provide immediate protection, no memory formation.

Definitions:

• Antigen: A molecule (often a protein) that induces an immune response; can be self or non-self.

• Antibody: A protein produced by B-cells that specifically binds to antigens to neutralize or mark them for destruction.

• Cytokines: Signaling molecules that facilitate communication between cells of the immune system.

Higher Order Concepts to Analyze and Apply

Activation of Helper and Cytotoxic T-cells:

• Helper T-cells:

  • Recognize antigens presented by MHC II molecules on APCs (e.g., macrophages).

  • Release cytokines that activate B-cells and cytotoxic T-cells.

• Cytotoxic T-cells:

  • Recognize infected cells presenting antigens via MHC I molecules.

  • Release perforins and granzymes leading to cell lysis.

Sequence of Events in Antibody-Mediated Immune Response:

1. Sensitization: B-cell binds to the specific antigen.

2. Activation: The B-cell presents the antigen on its MHC II and interacts with activated Helper T-cells.

3. Clonal Expansion: Activated B-cells proliferate and differentiate into plasma cells (producing antibodies) and memory B-cells.

4. Effector Phase: Antibodies are secreted, neutralizing pathogens or marking them for destruction.

Sequence of Events in Cell-Mediated Immune Response:

1. Recognition: Cytotoxic T-cells recognize infected cells via MHC I presenting antigens.

2. Activation: CD8 binds to MHC I, and additional signals from Helper T-cells activate Cytotoxic T-cells.

3. Killing: Cytotoxic T-cells release cytotoxic substances causing infected cells to undergo apoptosis.

Comparing and Contrasting Antibody- and Cell-Mediated Immune Responses:

• **Antibody-Mediated:

  • Involves B-cells and production of antibodies.

  • Effective against pathogens outside cells (extracellular pathogens).**

• **Cell-Mediated:

  • Involves T-cells, especially cytotoxic T-cells.

  • Targets infected or damaged cells (intracellular pathogens).**

Maturation of a T Cell and Important Membrane Components:

• Occurs in the thymus, where T-cells undergo positive and negative selection.

• Important components include T-cell receptors (TCR), CD4 (for Helper T-cells), and CD8 (for Cytotoxic T-cells).

Activation of a B-cell and Process of Antibody Production:

1. Antigen Binding: B-cell binds its specific antigen on its surface Ig receptors.

2. Presentation: Antigen is presented on MHC II, activating Helper T-cells.

3. Proliferation: B-cells proliferate into plasma cells and memory B-cells.

4. Antibody Production: Plasma cells secrete antibodies specific to the initial antigen, contributing to the immune defense.