innate immunity
Summary
Innate immunity is the body's first line of defense against pathogens, providing immediate (within minutes to hours), non-specific protection without prior exposure. It includes physical barriers, phagocytic cells, and pattern recognition receptors that respond to conserved microbial structures. Unlike adaptive immunity, it lacks immunologic memory but activates and shapes the adaptive immune response.
Detail
Innate immunity is the evolutionarily conserved, germline-encoded arm of the immune system that recognizes pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs), NOD-like receptors, and C-type lectin receptors. Components include: (1) Physical/chemical barriers - skin, mucous membranes, cilia, lysozyme, gastric acid; (2) Cellular components - neutrophils (first responders in acute inflammation), macrophages (phagocytosis, antigen presentation, cytokine release), dendritic cells (bridge to adaptive immunity), natural killer (NK) cells (kill virally infected/tumor cells via perforin/granzyme, no prior sensitization needed), eosinophils/basophils/mast cells (parasitic defense, allergic responses); (3) Soluble mediators - complement system (opsonization, membrane attack complex, chemotaxis), acute phase reactants (CRP, ferritin), cytokines (TNF-alpha, IL-1, IL-6 mediate fever and inflammation), interferons (antiviral state). Key clinical correlations: Neutrophil recognition of bacteria via TLRs triggers NF-kB pathway, leading to inflammatory cytokine production. Complement deficiencies (e.g., C5-C9) predispose to Neisseria infections. NK cell deficiencies increase susceptibility to herpesvirus infections. TLR4 recognizes LPS from gram-negative bacteria, crucial in sepsis pathophysiology. Innate immunity lacks memory (unlike adaptive immunity with T/B cells) but is essential for initiating and directing the adaptive immune response through antigen presentation via MHC molecules and costimulatory signals. This concept is frequently tested regarding immunodeficiency syndromes, hypersensitivity reactions, and the pathophysiology of sepsis/SIRS.
Sources
- First Aid for the USMLE Step 1
- Janeway's Immunobiology
- Robbins Basic Pathology
- Kaplan USMLE Step 1 Immunology
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