Overview
This lecture explains how the innate immune system defends the body using physical and chemical barriers, specialized cells, and inflammation to keep out and attack pathogens.
The Immune System: Overview
- The immune system protects the body from constant threats by microorganisms like bacteria and viruses.
- Unlike other organ systems, the immune system is made of widely distributed tissues, organs, and cells working together.
Innate (Nonspecific) Defense System
- The innate system is the first line of defense and responds immediately to pathogens.
- It includes barriers and cells present from birth that fight a broad range of invaders.
Physical and Chemical Barriers
- The skin acts as a tough barrier to prevent pathogen entry.
- Mucous membranes line body cavities exposed to the environment, providing additional protection.
- Stomach acid destroys swallowed pathogens.
- Mucus in the respiratory passages traps inhaled microbes.
- Saliva and tears contain enzymes that attack bacteria.
- Defensins are peptides in skin and membranes that inhibit bacteria and fungi.
Internal Innate Defenses
- If barriers are breached, the body activates internal defenses like fever, inflammation, and deployment of certain cells.
Phagocytes
- Phagocytes ingest and destroy invaders.
- Neutrophils are abundant white blood cells that engulf pathogens and then die (pus is dead neutrophils).
- Macrophages develop from monocytes, can consume many pathogens, and are either fixed or roaming in tissues.
Natural Killer (NK) Cells
- NK cells destroy infected or cancerous cells lacking the MHC1 protein marker.
- They kill by triggering apoptosis (programmed cell death) in abnormal cells.
Inflammatory Response
- Injury triggers mast cells to release histamine, causing redness, swelling, heat, and pain.
- Histamine dilates blood vessels and increases permeability, allowing fluid and proteins to enter injured tissue.
- Swelling helps form clots/scabs and allows immune cells easy access to the site.
- Neutrophils and monocytes/macrophages arrive to kill invaders and clean debris.
- Chemotactic signals attract immune cells to the site of infection.
Fever
- Widespread infection causes immune cells to release pyrogens, which raise body temperature.
- Fever speeds cell repair and reduces bacterial access to iron and zinc.
Key Terms & Definitions
- Innate Immunity — Nonspecific, immediate defense system present at birth.
- Phagocyte — Cell that engulfs and digests pathogens (e.g., neutrophil, macrophage).
- Neutrophil — Most common phagocytic white blood cell that dies after ingesting pathogens.
- Macrophage — Large phagocyte that can repeatedly consume pathogens.
- Natural Killer (NK) Cell — Immune cell that destroys infected or cancerous body cells.
- Histamine — Chemical released during inflammation causing vessel dilation and increased permeability.
- Pyrogen — Substance that induces fever by affecting the hypothalamus.
- MHC1 — Protein marker found on healthy cells; absent in infected/cancerous cells.
Action Items / Next Steps
- Review the steps of the innate immune response.
- Prepare to learn about the adaptive (specific) immune system in the next lecture.