innate immunity part 1
First Line Defenses
Definition and Importance
Always present defenses not turned on or off.
Skin is the largest immune organ and provides a physical barrier.
Skin Characteristics
Composed of multiple compact cell layers; outer layer consists of dead cells filled with keratin.
Keratin imparts toughness and waterproof qualities, preventing microbial penetration.
Some parasitic worms can burrow through the skin (exceptions).
Mucous Membranes
Internal surfaces exposed to the environment: gastrointestinal tract, respiratory tract, urogenital tract.
Continuous production of mucus by goblet cells provides a physical barrier that traps microorganisms.
Ciliary action in the respiratory tract helps move mucus loaded with microbes up to the mouth for swallowing.
Antimicrobial Factors
Saliva and Other Secretions
Contains antimicrobial substances like lysozyme, peroxidase, and lactoferrin.
Lysozyme breaks down bacterial cell walls; peroxidase generates toxic radicals; lactoferrin sequesters iron to inhibit microbial growth.
Stomach Defenses
Acidic pH (low) helps kill most microorganisms that survive the mouth.
Rapid pH change and normal microbiota further protect the gastrointestinal tract.
Normal Microbiota Role
Occupied space and consumed nutrients, preventing pathogen attachment.
Produced antimicrobial factors, adding to the protective effect.
Peristalsis: Continuous movement of food in the digestive tract helps flush out pathogens.
Urinary Tract Defense Mechanisms
Regular flushing mechanism prevents urinary tract infections.
Normal microbiota in the vagina further protect against infections.
Respiratory Tract Defense Mechanisms
Nasal passages trap microbes in mucus that is moved by cilia.
Mucociliary escalator carries microbes to the mouth for swallowing.
Macrophages present in the lungs can engulf any microbes that reach this depth.
Innate Immune Response Overview
Detection of Invaders
Sensors like complement proteins detect foreign cells and damage.
Innate Immune Cells
Understanding innate immune cells is key; these include neutrophils, macrophages, and dendritic cells.
Hematopoiesis and Immune Cells
Hematopoiesis
Occurs in bone marrow; progenitor cells differentiate into myeloid and lymphoid lines.
Key Immune Cells
Myeloid lineage: include red blood cells, platelets, neutrophils, monocytes, macrophages, dendritic cells.
Immune Response Recognition and Communication
Pattern Recognition Receptors (PRRs)
Recognize microbial patterns on pathogens or damage patterns on host cells.
Two groups of PRRs:
MAPs (Microbe-Associated Molecular Patterns): e.g., LPS, flagellin.
DAMPs (Damage-Associated Molecular Patterns): signal cell damage.
Cytokines as Communicators
Soluble signals released by cells (like macrophages) to inform other cells and initiate responses (e.g., inflammation).
Example cytokines: Interleukin-1 (induce fever), TNF (promote inflammation).
Phagocytosis Process
Key Phagocytes
Neutrophils: First responders that circulate in blood.
Macrophages: Tissue residents developed from monocytes.
Phagocytosis Steps
Recognition and attachment to pathogens.
Engulfment of pathogens into a membranous vesicle (phagosome).
Fusion of phagosome with lysosome to form a phagolysosome for digestion.
Elimination of microbial components via exocytosis.
Microbial Evasion Mechanisms
Some microbes produce substances such as enzymes and capsules to evade phagocytosis.
Others can survive within phagosomes and avoid lysosomal destruction.
Eosinophils in Response to Large Parasites
Eosinophils: Target larger eukaryotic parasites by attaching and releasing granules containing toxic products.
Elevated eosinophil counts in blood can indicate parasitic infections.
Summary
Small microbes are eliminated by phagocytosis while larger ones are attacked by eosinophils and other granulocytes. Next topics will cover the complement system and inflammatory responses.