Innate Immunity: Nonspecific Defenses of the Host (Chapter 16) Study Guide

Innate Immunity: Nonspecific Defenses of the Host

Overview of Innate vs. Adaptive Immune Response

The immune system protects the body from pathogens through two main types of responses: innate (nonspecific) and adaptive (specific) immunity. Innate immunity is the body's first line of defense and responds rapidly to a wide range of pathogens, while adaptive immunity is slower and targets specific pathogens.

• Innate Immunity: Present from birth, responds to all pathogens in a generic way, no memory.

• Adaptive Immunity: Develops after exposure, highly specific, involves memory cells (B and T lymphocytes).

B and T lymphocytes are not involved in innate immunity; they are key players in adaptive immunity.

First Line of Defense: Physical, Chemical, and Biological Barriers

The first line of defense includes mechanisms that prevent pathogens from entering the body. These are divided into physical, chemical, and biological barriers.

Physical Barriers

• Skin: Acts as a tough, impermeable barrier.

• Mucous membranes: Trap microbes and contain cilia to move them out.

• Mechanical actions: Such as coughing, sneezing, and the flow of urine, help remove pathogens.

Chemical Barriers

• Lysozyme: Enzyme found in tears, saliva, and mucus; breaks down bacterial cell walls.

• Lactoferrin: Protein in secretions and blood; binds iron, making it unavailable to microbes.

• Transferrin: Blood protein that also binds iron, limiting microbial growth.

• Defensins: Small antimicrobial peptides that disrupt microbial membranes.

Normal Flora (Microbiota)

• Protective Role: Compete with pathogens for nutrients and space.

• Competitive Exclusion: Prevent colonization by harmful microbes.

Components of Blood and Their Roles

Blood contains several cell types, each with distinct functions in immunity.

• Erythrocytes (RBCs): Transport oxygen; do not play a direct role in immunity.

• Platelets: Involved in clotting; help prevent pathogen entry through wounds.

• Leukocytes (WBCs): Main immune cells; some are phagocytic.

Types of Leukocytes

• Neutrophils (PMNs): Most abundant; highly phagocytic.

• Basophils: Release histamine; involved in inflammation.

• Eosinophils: Attack parasites; involved in allergic responses.

• Monocytes/Macrophages: Monocytes mature into macrophages; both are phagocytic.

• Dendritic Cells: Professional phagocytes; bridge innate and adaptive immunity.

• B and T Lymphocytes: Adaptive immunity only.

• Natural Killer (NK) Cells: Destroy infected or abnormal cells; part of innate immunity.

Professional phagocytic cells include neutrophils, macrophages, and dendritic cells.

Toll-Like Receptors (TLRs) and Cytokines

Toll-like receptors (TLRs) are proteins on immune cells that recognize pathogen-associated molecular patterns (PAMPs) and trigger immune responses.

• Function: Detect microbial components (e.g., lipopolysaccharide, flagellin).

• Result: Activation of immune cells and production of cytokines.

Cytokines are signaling proteins released by cells to communicate and regulate immune responses.

• Role: Coordinate inflammation, cell recruitment, and activation.

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

Phagocytosis: Process and Microbial Evasion

Phagocytosis is the process by which certain cells engulf and destroy pathogens.

• Stages:

  1. Chemotaxis: Movement toward pathogens.

  2. Adherence: Binding to pathogen.

  3. Ingestion: Engulfment into a phagosome.

  4. Digestion: Fusion with lysosome and destruction.

• Cells Involved: Neutrophils, macrophages, dendritic cells.

• Microbial Evasion Mechanisms:

  • Capsules (e.g., Streptococcus pneumoniae) prevent adherence.

  • Inhibition of phagosome-lysosome fusion (e.g., Mycobacterium tuberculosis).

  • Survival inside phagocytes (e.g., Salmonella).

• Opsonization: Complement proteins (e.g., C3b) coat pathogens, enhancing phagocytosis.

Inflammatory Response

Inflammation is a complex response to infection or injury, aiming to contain and eliminate pathogens and repair tissue.

• Signs: Redness, heat, swelling, pain, loss of function.

• Causes: Infection, tissue damage.

• Triggers: Release of cytokines and histamine.

• Steps:

  1. Vasodilation: Increased blood flow.

  2. Increased permeability: Fluid and immune cells enter tissue.

  3. Chemotaxis: Recruitment of phagocytes.

  4. Diapedesis: Leukocytes move out of blood vessels.

  5. Pus formation: Accumulation of dead cells and microbes.

  6. Tissue repair: Restoration of function.

• Cells Involved: Neutrophils (early), macrophages (later), basophils, eosinophils.

Vasodilation increases blood flow; diapedesis is the movement of leukocytes through vessel walls.

Fever

Fever is a systemic response to infection, characterized by an increase in body temperature.

• Mechanism: Cytokine IL-1 is released by phagocytes, acts on the hypothalamus to raise temperature.

• Benefits: Inhibits microbial growth, enhances immune activity.

Chemical Responses to Invading Organisms

Interferons

• Definition: Proteins produced by cells in response to viral infection.

• Induction: Triggered by viral nucleic acids.

• Effectiveness: Inhibit viral replication, activate immune cells.

Complement System

• Definition: A group of proteins in blood that enhance immune responses.

• Activation Pathways:

  1. Classical pathway

  2. Alternative pathway

  3. Lectin pathway

• Consequences/Functions:

  • Opsonization: C3b coats pathogens, enhancing phagocytosis.

  • Inflammation: C3a and C5a recruit immune cells.

  • Cytolysis: Membrane attack complex (MAC) formed by C5b, C6, C7, C8, C9 lyses cells.

Complement Protein	Function	Example
C3b	Opsonization	Enhances phagocytosis
C3a	Inflammation	Recruits immune cells
C5a	Inflammation, chemotaxis	Attracts phagocytes
C5b-C9	Cytolysis (MAC)	Lyses pathogen membranes

Membrane Attack Complex (MAC): Forms pores in pathogen membranes, leading to cell death.

Summary Table: Innate Immune Components

Component	Type	Role
Skin	Physical	Barrier to entry
Lysozyme	Chemical	Destroys bacteria
Normal Flora	Biological	Competitive exclusion
Neutrophils	Cellular	Phagocytosis
Complement	Chemical	Opsonization, inflammation, cytolysis

Example: Streptococcus pneumoniae uses a capsule to evade phagocytosis; C3b opsonization can help overcome this defense.

Additional info: The notes have been expanded to provide academic context, definitions, and examples for clarity and completeness.