Immune defense mechanisms against bacteria, viruses, fungi, and parasites, and common pathogen immune evasion strategies tested on USMLE Step 1.
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- What is the primary immune defense mechanism against extracellular bacteria?
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Opsonization and phagocytosis, along with complement activation and antibody-mediated neutralization- What immune cell type is the dominant early responder against acute extracellular bacterial infection?
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The neutrophil- What is the primary immune defense mechanism against intracellular bacteria such as Mycobacterium tuberculosis?
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Cell-mediated immunity, particularly macrophage activation by Th1 helper T cells via interferon-gamma- What is the hallmark tissue structure formed to contain intracellular bacteria like Mycobacterium tuberculosis?
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The granuloma- What immunodeficiency, involving defective IL-12/interferon-gamma signaling, leads to susceptibility to disseminated mycobacterial and atypical mycobacterial infection?
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Mendelian susceptibility to mycobacterial disease, from IL-12 receptor or interferon-gamma pathway defects- What is the primary immune defense mechanism against viral infections at the cellular level?
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Cytotoxic CD8+ T cells killing infected cells, along with type I interferon responses and NK cell activity- What cytokines are rapidly produced by virally infected cells to induce an antiviral state in neighboring cells?
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Type I interferons (interferon-alpha and interferon-beta)- What is the mechanism by which type I interferons induce an antiviral state?
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They upregulate genes that inhibit viral replication, such as those encoding proteins that degrade viral RNA or block protein synthesis- What antibody isotype is most important for neutralizing extracellular virus particles and preventing reinfection?
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IgG (and secretory IgA at mucosal surfaces)- What is the general immune defense strategy against fungal infections?
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A combination of neutrophils, macrophages, and Th17-driven cell-mediated immunity- What cytokine profile (Th17 pathway) is particularly important for defense against extracellular fungi and bacteria at mucosal and cutaneous surfaces?
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IL-17 and related Th17 cytokines- What immunodeficiency involving impaired Th17 responses (STAT3 mutation) causes recurrent skin and lung infections with characteristic eczema and elevated IgE?
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Hyper-IgE syndrome (Job syndrome)- What is the primary immune defense mechanism against multicellular helminth (parasitic worm) infections?
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IgE-mediated mast cell activation and eosinophil degranulation, driven by Th2 cytokines- What Th2 cytokines are especially important in driving IgE production and eosinophil recruitment against helminths?
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IL-4, IL-5, and IL-13- What immune defense mechanism is most important against protozoan parasites that live within macrophages, such as Leishmania?
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Th1-driven macrophage activation via interferon-gamma
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