Overview
This lecture covers the fundamentals of innate immunity, including key definitions, major defense lines, cell types, and mechanisms by which the body protects itself against pathogens in a nonspecific way.
Definitions and Immunity Types
- Immunity is the ability to ward off disease; susceptibility means lacking resistance.
- Innate immunity provides rapid, nonspecific defense against any pathogen and is present at birth.
- Adaptive (acquired) immunity targets specific pathogens, is slower, and has memory for future protection.
Lines of Defense
- First Line: Surface barriers like skin, mucous membranes, and secretions block pathogen entry.
- Second Line: Internal, nonspecific defenses like phagocytosis, inflammation, fever, and antimicrobial proteins.
- Third Line (Adaptive): Specific responses involving B and T cells, producing antibodies and memory.
First Line of Defense Components
- Physical Barriers: Intact skin (keratinized, shedding), mucous membranes (mucus traps microbes), cilia, sweat, urination, defecation, and reflexes (sneezing, coughing).
- Chemical Barriers: Sebum (lowers skin pH), lysozyme (breaks down bacterial cell walls), acidic gastric juice, vaginal secretions, antimicrobial substances in tears and semen.
- Microbiota: Normal flora compete with pathogens via microbial antagonism and alter local conditions to inhibit pathogens.
Second Line of Defense Components
- Phagocytosis: Carried out by neutrophils, monocytes/macrophages, dendritic cells, and eosinophils; involves chemotaxis, adherence, ingestion, digestion, and antigen presentation.
- Natural Killer (NK) Cells: Kill infected or cancerous self cells nonspecifically.
- Inflammation: Characterized by redness, pain, heat, and swelling; involves vasodilation, increased permeability, phagocyte migration (margination and diapedesis), and tissue repair.
- Fever: Raised body temperature induced by pyrogens inhibits microbial growth and enhances host defenses.
- Antimicrobial Proteins:
- Interferons: Released by virally infected cells to alert nearby cells and inhibit viral replication.
- Complement System: Serum proteins activate in a cascade to opsonize pathogens, induce cytolysis, and trigger inflammation.
Evasion of Innate Defenses
- Some microbes avoid phagocytosis by producing capsules, M proteins, killing phagocytes, preventing phagosome-lysosome fusion, or surviving digestion.
Key Terms & Definitions
- Innate Immunity — Nonspecific defense mechanisms present from birth.
- Adaptive Immunity — Specific immunity developed after exposure to pathogens.
- Phagocytosis — Process where cells ingest and destroy pathogens.
- PAMPs (Pathogen Associated Molecular Patterns) — Molecules on pathogens recognized as foreign.
- PRRs (Pattern Recognition Receptors) — Host cell receptors that detect PAMPs.
- Opsonization — Marking a pathogen to enhance phagocytosis.
- Cytolysis — Destruction of cells by bursting their membranes.
- Interferon — Protein that interferes with viral replication.
- Complement System — Group of proteins that assist in destroying microbes.
Action Items / Next Steps
- Review the roles and examples of physical, chemical, and microbiota barriers in the first line of defense.
- Understand the steps and outcomes of phagocytosis, inflammation, and the complement system.
- Read Chapter 13 for detailed coverage of adaptive immunity.