Host-pathogen interactions

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Headshot of Joanna Breems, MD, FACP · Clinical Assistant Professor
Joanna Breems
MD, FACP · Clinical Assistant Professor
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Table of Contents
High-yield summary

Core concept

  • Host-pathogen interactions determine infection outcomes: clearance, persistence, latency, or disease.

  • Immune responses are tailored to pathogen type

  • Pathogens evolve mechanisms to evade immunity.

 

Extracellular bacteria

  • Innate Response

    • Complement activation (Alternative and lectin pathways)

    • Phagocytosis via PRRs, Fc receptors, complement receptors

    • Inflammatory cytokines: IL-1, TNF, IL-6

  • Adaptive Response

    • Antibodies: Opsonization, toxin neutralization

    • CD4+ T cells (Th17): Recruit neutrophils

    • Th1: Activate macrophages

  • Adaptations

    • Capsules (mask PAMPs, resist opsonization)

    • Complement evasion: Bind Factor H, secrete proteases

    • Antigenic variation (e.g., Neisseria gonorrhoeae pili)

    • Biofilms: Protect from immune attack and antibiotics

Mnemonic for extracellular bacteria immune evasion: “Capsules Hide Bugs From Complement”
(Capsule, Factor H binding, Biofilm, Complement proteases)

 

Intracellular bacteria

  • Innate Response

    • IL-12 → NK cells → IFN-γ (activates macrophages)

    • Phagocytosis (often ineffective)

  • Adaptive Response

    • Th1 CD4+ cells: IFN-γ, IL-2 → macrophage activation

    • CD8+ CTLs: Minor role

  • *Key Cytokines

    • IFN-γ: Recruits and activates macrophages, enhances antigen presentation

    • TNF-a: Maintains macrophage activation = maintains integrity of granuloma

  • Adaptations

    • Block phagolysosome fusion (e.g., M. tuberculosis)

    • Escape to cytosol (e.g., Listeria ActA)

    • Mis-direct T-cell polarization (Th2/Treg bias)

 

Viruses

  • Innate Response

    • PRRs: TLR3 (dsRNA), TLR7/8 (ssRNA), TLR9 (CpG DNA)

    • Type I IFNs (α, β): Induce antiviral state (induce/suppress transcription → reduction of protein translation in virally infected cells; induce neighboring cells to increase anti-viral activities)

    • NK cells: Kill cells with ↓ MHC I

  • Adaptive Response

    • Antibodies: Block entry, neutralize extracellular virus

    • CD8+ CTLs: Kill infected cells

    • CD4+ helpers: Enhance CTL and Ab responses

  • Adaptations

    • Antigenic drift/shift (influenza)

    • Downregulate MHC I (evade CTLs; reliant then on NK Cells which are less effective for complete pathogen clearance)

    • Cytokine decoys (e.g., CMV)

    • Latency (e.g., Herpesviruses)

    • Direct immunosuppression (HIV infects CD4+ cells)

 

Fungi

  • Innate Response

    • Neutrophils critical against invasive disease (Neutropenia is greatest risk for invasive candidiasis)

    • PRRs: Mannan, β-glucan, C-type lectin

  • Adaptive Response

    • Th17: mucocutaneous

    • Th1: intracellular fungi → granulomas

  • Adaptations

    • Inhibit TNF and IL-12

    • Block phagolysosome function

 

Parasites

  • Innate Response

    • Eosinophils (major basic protein)

    • Complement (minor role)

  • Adaptive Response

    • IgE: mast cell activation

    • Th2: IL-4, IL-5, IL-13 → eosinophils, mucus, smooth muscle contraction

  • Adaptations

    • Antigenic variation (trypanosomes, malaria)

    • Concealment (intracellular or cysts)

    • Immune modulation: Treg induction, Ig proteases

Mnemonic: “Parasites Are Sneaky”
(Variation, Concealment, Suppression)

 

Immunopathology highlights

  • Cytokine storm: Multi-organ damage

  • Antibody-dependent enhancement: Dengue

  • Molecular mimicry: Acute Rheumatic Fever, Guillain-Barré post-viral infection

 

Step 1 high-yield associations

  • TB granulomas: Th1 + IFN-γ + TNF → caseating necrosis

  • Listeria: Actin-based motility

  • Neisseria gonorrhoeae: Antigenic variation → no vaccine

  • HIV: Direct immunosuppression

  • Helminths: IgE + eosinophils → Type I hypersensitivity

Learning goals

  1. Describe the mechanisms used by the immune system in response to extracellular bacteria and the mechanisms that extracellular bacteria use to evade immune responses
  2. Describe the mechanisms used by the immune system in response to intracellular bacteria and the mechanisms that intracellular bacteria use to evade immune responses
  3. Describe the mechanisms used by the immune system and viruses in viral infection and their roles in viral-induced immune pathology
  4. Describe the mechanisms used by the immune system in response to fungal and parasitic infections and the mechanisms that fungi and parasites use to evade immune responses

In as much as normal human physiology tends towards homeostasis through the constant response to and balancing of forces, homeostasis with our external, molecular environment is always happening. The microbiologic organisms in and around us are in dynamic interplay that influence disease and health, and drive evolutionary changes in both host and microorganism. Understanding host-pathogen interactions is important in understanding how we prevent and treat infectious diseases. In this session, we will explore the immune system responses to different types of pathogens and the strategies pathogens use in response.

These are not required; they are supplementary to the large-group session. They are intended as a curated guide to content focused on the learning objectives. There are both textbook and video resources for this session for students to use per their preference.

For each reference, I have designated the learning goal addressed with a learning goal icon and and number.

Click the  blue icons below to go to the resources listed.

IMMUNOLOGY: You will need to recall the details learned in immunology curriculum to achieve the learning objectives.

  • Sections on Host Factors, Host Defenses, Adaptive Immune Responses, Virus-Induced Immunopathology, and Virus-induced immunosuppression
  • Section of Attributes of Bacterial Pathogenicity
  • Sections on Pathogenesis and Immunity
  • Sections on Pathogenesis and Immunity and Immune Evasion

  • Check your learning

    Compare and contrast the innate and adaptive responses for each microorganism. (Take a screenshot and fill it in using your favorite note-taking application.)

 

Major Innate Response (cells, receptors, cytokines)

Major Adaptive Response (cells, receptors, cytokines)

Extracellular Bacteria

 

 

Intracellular Bacteria

 

 

Viruses

 

 

Fungi

 

 

Parasites

 

 

  • Check your learning

    Fill in the methods of adaptation and key examples for each microorganism. (Take a screenshot and fill it in using your favorite note-taking application.)

 

Methods of Adaptation

Key Examples

Extracellular Bacteria

 

 

Intracellular Bacteria

 

 

Viruses

 

 

Fungi

 

 

Parasites