Innate and Adaptive Immunity: Phagocytic Cells, Inflammation, and Immune Response

Innate Immunity and Phagocytic Cells

The Innate Immune System

• Definition: First line of defense against pathogens.
• Characteristics: Rapid, non-specific response.

Phagocytic Cells: Generalists of the Innate Immune System

• Function: Engulf and destroy microorganisms.
• Recognition: Identify broad classes of pathogens, not specific types.

Types of Phagocytic Cells

Macrophages

• Function: Large eaters ("macro" = large, "phage" = eat).
• Location: Resident in tissues as first responders.
• Role in Inflammation: Key initiator by releasing signaling molecules.

Neutrophils

• Function: Engulf and destroy microorganisms.
• Recruitment: Mobilized from blood to infection sites by macrophages.
• Lifespan: Short-lived, die at infection site after completing their function.

Dendritic Cells

• Phagocytic Capacity: Capable of engulfing pathogens.
• Main Role: Process engulfed pathogens for presentation to adaptive immune cells.
• Location: Reside in tissues like macrophages.

Pattern Recognition Receptors (PRRs)

Characteristics of PRRs

• Generalist Recognition: Enable broad pattern recognition of pathogens.
• Location: Found throughout phagocytic cells
  • Plasma Membrane: Detect extracellular pathogens.
  • Internal Membranes: Detect internalized pathogens.
  • Cytoplasm: Detect pathogens that have escaped into the cytosol.
• Necessity for Diverse Locations: Different pathogens have various lifestyles and structures, necessitating diverse PRRs to ensure defense effectiveness.

Toll-Like Receptors (TLRs)

• Definition: A specialized PRR recognizing distinct molecular patterns associated with pathogens.

Diversity of TLRs

• Types: Numerous types exist (e.g., TLR1-TLR10 in humans).
• Specificity in Generalism: While generalists, each TLR recognizes specific molecular patterns.

Examples of TLRs

• TLR4: Recognizes Lipopolysaccharide (LPS) from Gram-negative bacteria; not found in human cells.
• TLR5: Recognizes Flagellin, a bacterial flagella component.
• TLR3: Present in internal membranes; recognizes double-stranded RNA (dsRNA), typical of certain viruses.
• TLR9: Present in internal membranes; recognizes unmethylated CpG DNA, distinguishing it from human DNA which is typically methylated.

Phagocytosis: The Process of Engulfment and Destruction

1. Engulfment: Membrane of phagocyte extends around the pathogen, forming a phagosome.
2. Fusion with Lysosome: Phagosome fuses with lysosome creating a phagolysosome.
3. Destruction: Digestive enzymes within phagolysosome break down the pathogen.
4. Elimination: Pathogen is destroyed to prevent replication and spread.

Initiation of the Inflammatory Response

• Signaling Molecules Released: Include cytokines and chemokines to initiate inflammation and recruit more immune cells.

Types of Signaling Molecules

• Cytokines: Signaling molecules affecting nearby cells.
• Chemokines: Specific type of cytokine guiding cell movements (chemotaxis).

Examples of Signaling Molecules

• Mast Cells: Release histamine.
• Macrophages: Release TNF-alpha.

Cellular Players and Their Actions

• Macrophages: Detect pathogens via PRRs and release signals to recruit immune cells.
• Mast Cells: Release signaling molecules like histamines.
• Endothelial Cells: Line capillaries; responsive to immune signals, facilitate immune cell recruitment.

Changes in Capillaries During Inflammation

Specific Changes Triggered by Immune Signals

1. Vasodilation: Increased diameter of capillaries; enhances blood flow and aids cell exit.
2. Increased Permeability: Junctions between endothelial cells open, leading to fluid and protein leakage.
   • Fluid Leakage: Contributes to swelling.
   • Protein Leakage: Important immune proteins leave into the tissue.

Proteins That Leak Out

• Antibodies: Tag pathogens for destruction if specific.
• Complement Proteins: >20 non-specific innate immune proteins involved in pathogen defense.
  • Membrane Attack Complex (MAC): Pore formation in pathogen membranes, leading to lysis.
  • Opsonization: Tagging pathogens for removal by phagocytic cells.

Recruitment of Immune Cells (Diapedesis)

• **Mechanism Outline: **

1. Upregulation of Adhesion Receptors: Endothelial cells express new receptors in response to inflammatory signals.
2. Rolling Adhesion: Weak receptor binding slows neutrophils, causing them to roll along capillary walls.
3. Tight Binding: Further signaling facilitates strong binding to adhesion molecules (e.g., ICAM-1), halting rolling motion.
4. Diapedesis (Extravasation): Neutrophils squeeze between endothelial cells into the interstitial tissue.
5. Chemotaxis: Neutrophils follow chemical gradients to the infection site.

Hallmarks of Inflammation

Observable Signs of Inflammation

• Heat and Redness: Due to increased blood flow from vasodilation.
• Swelling: Result of fluid leakage from capillaries.
• Pain: Caused by pressure on nerve endings from swelling and irritants from pathogens./

Neutrophil Extracellular Traps (NETs)

• Definition: Structures formed when dying neutrophils release chromatin (DNA and associated proteins).
• Function: Trap and kill bacteria and other pathogens.

Connecting Innate and Adaptive Immunity

• The innate immune system can activate the more specific, long-lasting adaptive immune system.

Dendritic Cells as a Bridge

• Engulfment: Capture pathogens using PRRs.
• Migration: Travel to secondary lymphoid organs through lymphatic system after engulfment.
• Antigen Presentation: Processed pathogen fragments presented on their surface using MHC molecules along with co-stimulatory signals.
• T Cell Activation: Presented antigens activate T cells within lymph nodes.

The Lymphatic System

• Definition: A network of vessels and organs vital for immune surveillance and maintaining fluid balance.

Functions of the Lymphatic System

• Fluid Drainage: Returns interstitial fluid leaking from blood capillaries.
• Immune Cell Transport: Moves immune cells like lymphocytes and antigen-presenting cells.
• Lymph Nodes: Meeting places for immune cells to interact (like B cells, T cells, dendritic cells).

Primary vs. Secondary Lymphoid Organs

Classification of Lymphoid Organs

• Primary Lymphoid Organs (Development): Sites where lymphocytes develop and mature.

  • Bone Marrow: Source of all blood cells, including lymphocytes, from stem cells. Site for B cell development.
  • Thymus: Location where T cells migrate from the bone marrow for maturation.

• Secondary Lymphoid Organs (Activation): Sites for mature lymphocytes to encounter antigens and activate.

  • Lymph Nodes: Filter lymph and facilitate antigen presentation.
  • Spleen: Functions similarly to lymph nodes for blood-borne pathogens.
  • MALT (Mucosa-Associated Lymphoid Tissues): Protect mucosal surfaces (e.g., tonsils, Peyer's patches).

Adaptive Immunity: Key Traits and Specificity

• Definition: Highly specific, diverse, memory-driven immune response.

Key Traits of the Adaptive Immune System

1. Specificity: Each B or T cell expresses antigen receptors with unique epitopes.
2. Diversity: A vast array of B and T cells with different receptor specificities are present.
3. Self vs. Non-Self Recognition (Tolerance): Distinguishes between body's own cells and foreign invaders; prevents autoimmunity.
4. Memory: Produces long-lived memory cells after primary exposure, enabling faster response upon re-exposure (vaccination).

Antigen Receptors: B Cell Receptors (BCRs) and T Cell Receptors (TCRs)

• Antigen Definition: A substance eliciting a specific immune response from B or T cells.
  • Epitope: Recognized small region on an antigen by antigen receptors.

B Cell Receptors (BCRs)

• Structure: Composed of four polypeptide chains (two heavy, two light), forming a Y-shaped molecule.
• Antigen Binding: Each BCR is bivalent, binding two identical epitopes.
• Secretion (Antibodies): Upon activation, B cells become plasma cells secreting soluble BCR forms known as antibodies.
• Binding Capacity: Can bind diverse free-floating or surface-bound antigens.

T Cell Receptors (TCRs)

• Structure: Typically consists of two polypeptide chains (alpha and beta).
• Antigen Binding: TCRs recognize presented antigens on MHC molecules; they cannot bind to free-floating antigens.

Generation of Antigen Receptor Diversity (V(D)J Recombination)

Mechanism of Diversity Creation

1. Gene Segments: Antigen receptor genes made of multiple segments (V, D, J, C).
2. Random Selection: Enzymes select segments randomly during lymphocyte development.
3. DNA Deletion: Non-selected DNA sequences are cut out permanently.
4. Sloppy Joining: Junctional diversity adds variability to resulting polypeptide chains but may shift the reading frame.
5. Combinatorial Diversity: The combination of unique segments creates a massive pool of potential antigen receptors.

Structure of B Cell Receptor Chains

• Light Chain: Formed from V-J recombination.
• Heavy Chain: Formed from V-D-J recombination.

Constant Regions (C Regions)

• Location: Found in both heavy and light chains after the variable region.
• Function: Structural roles dictate antibody class and function; limited diversity compared to variable regions.

Self-Tolerance and Lymphocyte Maturation

Selection Process to Prevent Autoimmunity

1. Testing Location: Occurs in primary lymphoid organs (bone marrow for B cells, thymus for T cells).
2. Process: Lymphocytes tested against self-antigens.

• Strong Self-Recognition: Dangerous receptors bind strongly to self, leading to:
  • Deletion: Most self-reactive cells undergo apoptosis.
  • Rearrangement: Some cells may attempt to rearrange receptors.
  • Anergy: Some self-reactive cells may become non-functional.

Naive Lymphocytes

• Definition: Successfully tested lymphocytes awaiting specific antigen encounters.
• Activity: Circulate through blood and lymph, patrolling secondary lymphoid sites.

Activation and Clonal Expansion

• Process Overview: When naive B or T cells meet their antigen, they undergo activation.
• Clonal Expansion: Rapid proliferation produces a large clone of genetically identical cells specific to the pathogen.
• Differentiation: Cells differentiate into effector and memory cells:
  • Plasma Cells: Specialized B cells producing large quantities of antibodies.
  • Memory Cells: Long-lived cells that ensure rapid response on subsequent exposures.

Memory Cell Function in Immune Response

• Plasma Cells: Become antibody-secreting factories leading to immediate immune response.
• Memory Cells: Persist for years, allowing for immediate reinfection responses, crucial for long-term immunity and vaccinations.

Primary vs. Secondary Immune Response

**Primary Response: **

• Characteristics: First pathogen encounter, initial slow antibody production (6-7 days), lower effector cell numbers.

**Secondary Response: **

• Rapid memory cell activation leads to quicker, stronger responses, larger effector populations, peak response time about one week.

Vaccination Strategy

• Goal: Stimulate memory cell production without full disease risk by introducing:
  • Pathogen pieces
  • Killed pathogens
  • Attenuated pathogens
• Limitations: Recognition depends on pathogen structure; mutations may prevent recognition.

Humoral Immune System: B Cells Overview

• B Cell Receptor (BCR) Structure:
  • 4 polypeptide chains (2 heavy, 2 light).
  • ~100,000 receptors per cell with unique specificity.

B Cell Activation Process

• Upon Activation: Naive B cells convert to plasma cells, producing 5000 antibodies per second, matching the original BCR specificity.
• Key Concept: B cells and antibodies offer targeted immune responses via precise antigen interaction.

Antibody Functions

• Antibody Capabilities:
  • Plasma cells secrete thousands of antibodies, have limited lifespan.
• Key Antibody Mechanisms:
  1. Phagocyte Recognition Optimization: Enhances phagocytic effectiveness.
  2. Pathogen Neutralization: Blocks pathogen activity primarily for viruses.
  3. Complement System Activation: Triggers cascades leading to pathogen destruction.

Antibody Structure and Classes

• Classes:
  • IgG: Most abundant, activates complement.
  • IgM: Pentamer structure, multiple binding sites, activates complement.
  • IgA: Present in secretions (tears, saliva).
  • IgE: Involved in allergies, found on mast cell surfaces.
  • IgD: Less understood.
• Structural Notes:
  • Class distinction based on constant region sequences.
  • Antigen binding through variable regions.

B Cell Activation and Differentiation

• Naive B Cells: Non-antibody producing; interact with circulating antigens.
• Selection Process: Ensures targeted activation, preventing unnecessary expansions, leading to efficient responses.

Plasma Cells: The Antibody Factory

• Characterization: High rough endoplasmic reticulum (RER) specialized for protein secretion, producing 5000 antibodies/second.

B Cell Activation Mechanism: Steps

1. Antigen binds to BCR.
2. Internalization occurs.
3. Breaks down in lysosomes.
4. Bound to MHC Class II molecules and presented on the surface.
5. T cells stimulate B cell proliferation via activating cytokines.

B Cell Differentiation Outcomes

• Primary Products:
  • Plasma Cells: Immediate antibody production, short-lived.
  • Memory B Cells: Long-lived, promoting immunological memory.

Primary vs. Secondary Immune Responses

Primary Immune Response Characteristics

• Initial antigen exposure leads to a ~6-7 day lag before the first antibodies appear, peaking up to 12 days.

Secondary Immune Response Dynamics

• Accelerated antibody production, higher concentrations, and quicker peak (about one week), due to memory based activation.

Memory Cell Significance:

• Provide long-term immunity, rapidly respond to reinfection, essential for ongoing protection.

Vaccination Strategy

• Main Objective: Generate memory cells for B cells and helper T cells; allows the quick response to the actual pathogens without risk of disease.

Immune System's Arms Race: Pathogen Evasion Strategies

Staphylococcus aureus Example

• IgA Interaction: Binds to pathogens, enabling macrophage engagement for destruction.
• SSL7 Protein: Blocks engagement with antibodies, antibodies unable to opsonize, interferes with complement recognition.

Antibody Effector Mechanisms

Primary Functions:

1. Neutralization of pathogens.
2. Agglutination of microbes to facilitate macrophage capture.
3. Precipitation of antigens.
4. Complement activation and MAC formation.

Primary Immune Response Mechanism

• Initial exposure triggers APCs and T/B cell activation.
• Involves stimulatory interactions and memory cell generation.

Secondary Immune Response Activation

• Memory cells reactivated, leading to rapid T/B cell stimulation, resulting in improved immune mechanisms.

Types of Immunity: Active vs. Passive

Active Immunity

• Mechanisms: Natural infection or vaccination generating long-lasting memory cells.

Passive Immunity

• Provide Immediate Protection: Natural sources (maternal antibodies) or therapeutic (monoclonal antibodies).

Monoclonal Antibodies (MAbs)

Definition and Production

• Derived from a single clone, produced through hybridoma technology for specific antigen targeting.

Applications of Monoclonal Antibodies

• Treatment for infectious diseases and for cancer.
• Protective mechanisms include opsonization and complement activation.

Limitations of Passive Immunity

• Short-lived without long-term memory; relies on administered antibody levels.

Vaccination: Active Immunity Form

• Generates protective responses without pathogen exposure risks.
• Principles include safety, efficacy, stability, and ease of administration.

Types of Vaccines

• Attenuated: Weakened strains allowing for immune recognition.
• Killed Pathogens: No replication capability - zero disease risk.
• Pathogen Components: Including proteins, polysaccharides, and inactivated toxins.
• Advanced Techniques: Include conjugate, viral vector, and nucleic acid vaccines.

COVID-19 Vaccine Mechanism and Development

• Nucleic acid introduced to stimulate immune responses without introducing the virus itself.
• Acknowledging the importance of booster shots due to evolving viral strains.

Immunity, Viruses, and Cancer

Viral Connections to Cancer

• 15-20% of human cancers are linked to viral infections.
• The immune system is critical in preventing cancer progression, and some immunodeficient individuals face increased cancer risks.

Human Papillomavirus (HPV) Case Study

• Associated with multiple cancers; the Gardasil vaccine prevents significant strains related to cervical and other cancers.