Innate Immunity: Defense Mechanisms of the Host (Ch. 12) – Study Guide Notes

Innate Immunity

Defense Mechanisms of the Host

Host protection against microbial invasion is organized into three lines of defense. The first line consists of physical and chemical barriers, the second line includes cellular and molecular responses, and the third line is the adaptive immune system. This chapter focuses on the innate immune system, which encompasses the first and second lines of defense.

• First Line of Defense: Physical and chemical barriers that prevent entry of pathogens. Examples include skin, mucous membranes, and secretions.

• Second Line of Defense: Non-specific cellular and molecular responses, such as phagocytosis, inflammation, and fever.

• Third Line of Defense: Adaptive immunity, involving lymphocytes and the production of antibodies (covered in later chapters).

Example: The skin acts as a physical barrier, while lysozyme in tears provides a chemical barrier against bacteria.

The Second and Third Lines of Defense: Overview

Surveillance Systems and Self vs. Nonself Recognition

The immune system distinguishes between self and nonself using markers on cell surfaces. This recognition is essential for mounting an effective response against pathogens while avoiding damage to host tissues.

• Self markers: Molecules present on host cells that identify them as "self" to the immune system.

• Nonself markers: Molecules on pathogens or foreign cells that trigger immune responses.

• Key components: Mononuclear phagocyte system (MPS), extracellular fluid, lymphatic system, and blood.

Mononuclear Phagocyte System (MPS)

The MPS is a network of connective tissue fibers and phagocytic cells (macrophages) that provide a passageway for immune cells throughout the body. It is closely associated with the lymphatic system and blood vessels.

• Function: Supports immune cell movement and communication.

• Cells involved: Macrophages, dendritic cells, and other phagocytes.

Extracellular Fluid (Interstitial Fluid)

This fluid surrounds tissue cells and bathes all cells, allowing for the movement of immune cells and molecules between blood and lymph vessels.

Lymphatic System

The lymphatic system returns interstitial fluid to the general circulation and is involved in immune surveillance. It includes lymph nodes, spleen, thymus, and lymphatic vessels.

• Function: Filters lymph, traps pathogens, and facilitates immune cell activation.

Blood

Blood contains both specific/adaptive and nonspecific/innate immune defenses. It transports immune cells and molecules throughout the body.

• Components: Plasma (fluid portion), erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

The Second Line of Defense

Phagocytosis and Phagocytic Cells

Phagocytosis is the process by which certain immune cells engulf and destroy pathogens. The main phagocytic cells are neutrophils, macrophages, and dendritic cells.

• Neutrophils: Most abundant phagocytes; respond quickly to infection.

• Macrophages: Derived from monocytes; can reside in tissues and become activated during inflammation.

• Dendritic cells: Specialized for antigen presentation to T cells.

Example: Neutrophils migrate to sites of infection and engulf bacteria, destroying them with enzymes and reactive oxygen species.

Inflammation

Inflammation is a non-specific response to tissue injury or infection, characterized by redness, heat, swelling, and pain. It helps contain infections and promotes healing.

• Steps: Vasodilation, increased permeability, migration of phagocytes, and tissue repair.

• Mediators: Cytokines, chemokines, and other inflammatory molecules.

Pattern Recognition and Pathogen-Associated Molecular Patterns (PAMPs)

Innate immune cells recognize PAMPs, which are molecular signatures found on pathogens but not on host cells. Recognition occurs via pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs).

• PAMPs: Examples include lipopolysaccharide (LPS) from Gram-negative bacteria, peptidoglycan from Gram-positive bacteria, and viral double-stranded RNA.

• PRRs: Receptors on phagocytes and other immune cells that detect PAMPs and initiate immune responses.

Cytokines

Cytokines are signaling proteins released by immune cells to coordinate responses. They can be pro-inflammatory or anti-inflammatory and have pleiotropic effects.

• Examples: Interleukins (ILs), interferons (IFNs), and tumor necrosis factor (TNF).

Fever

Fever is an increase in body temperature that enhances immune function and inhibits pathogen growth. It is triggered by pyrogens, which can be endogenous (from host cells) or exogenous (from pathogens).

• Benefits: Inhibits microbial growth, increases metabolism, and promotes healing.

Antimicrobial Proteins

These proteins help destroy pathogens and include interferons, complement proteins, and antimicrobial peptides.

• Interferons: Cytokines that interfere with viral replication and activate immune cells.

• Complement system: A group of proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.

• Antimicrobial peptides: Small proteins that disrupt microbial membranes.

Natural Killer (NK) Cells

NK cells are lymphocytes that target and destroy virus-infected and cancerous cells by inducing apoptosis.

• Function: Provide rapid responses to infected cells and produce cytokines.

Summary Table: Key Components of Innate Immunity

Component	Function	Example
Physical Barriers	Prevent pathogen entry	Skin, mucous membranes
Chemical Barriers	Destroy or inhibit pathogens	Lysozyme, acidic pH
Phagocytes	Engulf and destroy pathogens	Neutrophils, macrophages
Cytokines	Coordinate immune responses	Interleukins, interferons
Complement System	Lyse pathogens, enhance phagocytosis	C3, C5 proteins
NK Cells	Destroy infected/cancerous cells	Induce apoptosis

Key Equations and Concepts

• Opsonization: The process by which pathogens are marked for phagocytosis by the binding of opsonins (e.g., antibodies, complement proteins).

• Inflammatory Response: Sequence of events:

  • Vasodilation

  • Increased vascular permeability

  • Migration of phagocytes

  • Tissue repair

• Complement Activation: Can occur via three pathways:

  • Classical pathway (antibody-dependent)

  • Alternative pathway (pathogen surface-dependent)

  • Lectin pathway (mannose-binding lectin)

Equation for Complement Activation (Simplified):

$C3 \xrightarrow{activation} C3a + C3b$

Equation for Phagocytosis (Simplified):

$\text{Phagocyte} + \text{Pathogen} \rightarrow \text{Phagocyte-Pathogen Complex} \rightarrow \text{Destruction of Pathogen}$

Additional info:

• Adaptive immunity (third line of defense) involves specific recognition of antigens and memory formation, which is covered in later chapters.

• Some cytokines, such as interferons, have both antiviral and immunomodulatory effects.