Innate Immunity and Immune Responses

BSC 283/283L Announcements

• Chapter 11 Pearson HW due Monday April 6
• Dynamic Study Module for Chapter 11 has been posted

Microbiology Basic and Clinical Principles - Chapter 11: Innate Immunity

Overview of the Immune System and Responses

Learning Objectives

• Describe the general features of innate and adaptive immunity.
• Describe how normal microbiota may impact immune responses and limit pathogens.

Immune Responses Classifications

• Immune responses are classified as either innate or adaptive.
• An immune response is a physiological process coordinated by the immune system to eliminate antigens.
• The immune system includes two key branches:
    - Innate Immunity
    - Adaptive Immunity
• Three common features of both immune types:
    1. Recognize diverse pathogens
    2. Eliminate identified invaders
    3. Discriminate between self and foreign antigens

Innate Immunity

• Innate immunity is inborn and provides ancient protection existing in some form across all eukaryotic organisms.
• It offers generalized responses and is characterized by:
    - Nonspecific immunity
    - Immediate response upon encountering pathogens

Adaptive Immunity

• Adaptive immunity occurs only in vertebrate animals.
• It matures over time and reacts with tailored responses to pathogens, typically requiring 4–7 days to activate fully.
• It displays memory through prior exposure, which is the foundational principle behind how vaccines work.

Definitions

• Immune: Specific protection conferred by adaptive immune responses.
• Susceptible: Not immune to a given pathogen, which may lead to infection.

Comparative Features of Innate vs. Adaptive Immunity

Collaboration Between Innate and Adaptive Immunity

• Immunity consists of three collaborating lines of defense:
    1. Barrier defenses (in the innate immunity)
    2. Cellular and molecular defenses
    3. Adaptive defenses

Role of Normal Microbiota in Immune Response

• Many microbes live as symbiotic partners on our bodies, excluding pathogens.
• These microbes help fine-tune the immune system to fight pathogens while teaching tolerance toward non-pathogens, food, and self-tissues.
• Alterations in normal microbiota may lead to immune system confusion, causing overreaction against harmless agents—linking shifts to increased allergy and autoimmunity rates.

Hygiene Hypothesis

• The hygiene hypothesis proposes that decreased diversity and levels of microbes in our normal microbiota may negatively affect immune responses.
• Research on the microbiota's impact on immunity is still in its infancy.

Introduction to First-Line Defenses

Learning Objectives

• State the general function of first-line defenses.
• Describe mechanical, chemical, and physical barriers with examples.
• Discuss what makes skin a useful barrier.
• Explain lysozyme and its functions and locations.
• Describe the role of antimicrobial peptides.

First-Line Defenses

• First-line defenses attempt to prevent pathogen entry and are categorized as:
    - Mechanical barriers
    - Chemical barriers
    - Physical barriers

Mechanical Barriers

• Mechanical barriers rinse, flush, or trap pathogens:
    - Tears: wash debris/pathogens from the eyes
    - Urine: flushes microbes out of the body
    - Saliva: limits microbial adherence to oral tissues
    - Mucous membranes: trap microbes, lining all body entrances and internal surfaces of the stomach, intestines, lungs, and bladder
    - Earwax: traps microbes
    - Mucociliary escalator: removes mucus from the lungs towards the mouth via ciliated cells

Chemical Barriers

• Chemical barriers directly attack invaders or create unfavorable environments for pathogens:
    - Lysozyme: Found in secretions (e.g., tears, breast milk) that degrade bacterial cell walls
    - Hydrochloric acid in the stomach
    - Skin: generally dry, salty, and slightly acidic
    - Fatty acids in sweat and earwax

Antimicrobial Peptides (AMPs)

• AMPs: Proteins that destroy a broad spectrum of pathogens (viruses, parasites, bacteria, fungi):
    1. Stimulate leukocytes and modulate inflammation
    2. Directly target pathogens:
       - Disrupt plasma membranes and cell walls
       - Target intracellular components
    - Defensins: A type of mammalian AMP that kills invaders by inserting into target cell membranes

Physical Barriers

• Physical barriers physically block pathogen entry:
    - Epithelial tissue: Primary physical barrier in animals, lining all cavities and body entrances
    - Skin: A significant physical barrier composed of tightly compacted dead epithelial cells enriched with proteins and lipids, serving as a water-resistant layer

Introduction to Second-Line Defenses and the Lymphatic System

Learning Objectives