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

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 present from birth and provides immediate, general protection, while adaptive immunity develops over time and targets specific pathogens.

• Innate Immunity: Rapid, nonspecific, no memory. Includes physical, chemical, and cellular defenses.

• Adaptive Immunity: Slower, highly specific, has memory. Involves B and T lymphocytes.

Example: Skin acts as a physical barrier (innate), while antibodies produced after vaccination are part of adaptive immunity.

First Line of Defense: Physical and Chemical Barriers

The first line of defense prevents pathogens from entering the body. It includes mechanical and chemical mechanisms.

• Physical Barriers:

  • Skin: Tough, impermeable outer layer.

  • Mucous membranes: Trap microbes; cilia move them out.

  • Tears, saliva, urine: Wash away pathogens.

• Chemical Barriers:

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

  • Lactoferrin: Binds iron, making it unavailable to microbes.

  • Transferrin: Similar to lactoferrin; found in blood and tissue fluids.

  • Defensins: Antimicrobial peptides that disrupt microbial membranes.

• Normal Flora:

  • Compete with pathogens for nutrients and space (competitive exclusion).

  • Produce substances that inhibit pathogens.

Example: Staphylococcus epidermidis on skin prevents colonization by harmful bacteria.

Components of Blood and Their Roles

Blood contains cells and proteins essential for immune defense.

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

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

• Leukocytes (WBCs): Main immune cells. Types include:

Cell Type	Main Function	Phagocytic?
Neutrophils (PMNs)	Phagocytosis; first responders	Yes
Basophils	Release histamine; inflammation	No
Eosinophils	Attack parasites; allergic response	Somewhat
Monocytes/Macrophages	Phagocytosis; antigen presentation	Yes
Dendritic Cells	Phagocytosis; antigen presentation	Yes
B Lymphocytes	Antibody production (adaptive)	No
T Lymphocytes	Cell-mediated immunity (adaptive)	No
NK Cells	Kill infected/tumor cells	No

Note: B and T lymphocytes are not involved in innate immunity.

Toll-Like Receptors (TLRs) and Cytokines

Toll-Like Receptors (TLRs) are proteins on immune cells that recognize pathogen-associated molecular patterns (PAMPs), such as bacterial cell wall components or viral RNA.

• Trigger immune responses upon detection of pathogens.

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

• Coordinate inflammation, cell recruitment, and activation.

• Examples: Interleukins (ILs), interferons.

Phagocytosis: Process and Evasion Mechanisms

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

• Stages of Phagocytosis:

  1. Chemotaxis: Movement toward microbes.

  2. Adherence: Binding to microbe.

  3. Ingestion: Engulfment into a phagosome.

  4. Digestion: Fusion with lysosome; destruction of microbe.

• Cells Involved: Neutrophils, macrophages, dendritic cells.

• Microbial Evasion:

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

  • Some bacteria survive inside phagocytes (e.g., Mycobacterium tuberculosis).

• Opsonins: Molecules (e.g., C3b) that enhance phagocytosis by marking microbes.

Inflammatory Response

Inflammation is a localized 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: WBCs and proteins enter tissues.

  3. Chemotaxis: WBCs migrate to site.

  4. Phagocytosis: Removal of pathogens.

  5. Pus formation: Accumulation of dead cells.

  6. Tissue repair: Restoration of function.

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

• Key Terms: Vasodilation (widening of blood vessels), diapedesis (movement of WBCs out of blood vessels).

Fever

Fever is a systemic response to infection, characterized by increased body temperature.

• Triggered by cytokines (e.g., IL-1) acting on the hypothalamus.

• Benefits: Inhibits microbial growth, enhances immune activity.

Chemical Responses: Interferons and Complement System

Interferons

Interferons are proteins produced by cells in response to viral infection.

• Induced by viral presence.

• Effectiveness: Inhibit viral replication, activate immune cells.

Complement System

The complement system is a group of proteins in blood that enhance immune responses.

• Three activation pathways: Classical, alternative, lectin.

• Functions:

  • Opsonization: C3b coats microbes, enhancing phagocytosis.

  • Inflammation: C3a and C5a recruit immune cells.

  • Cytolysis: Membrane attack complex (MAC) forms pores in microbial membranes.

Complement Protein	Function
C3b	Opsonization
C3a	Inflammation
C5a	Inflammation, chemotaxis
MAC (C5b-C9)	Cytolysis

Example: C3b binds to bacteria, making them easier for phagocytes to ingest.

Additional info: These notes expand on brief points by providing definitions, examples, and context for each mechanism and cell type involved in innate immunity, as well as the complement system and its functions.