Overview
This lecture explains the human immune system, outlining its three lines of defense, types of immunity, and the key cells and molecules involved in immune responses.
Types of Immunity
- Immunity protects the body from foreign substances or pathogens.
- Passive immunity occurs when antibodies are given to a person (e.g., via breast milk or antibody serum).
- Passive immunity is short-term and does not require exposure to pathogens.
- Active immunity develops when the body is exposed to an infection or receives a vaccine.
- Natural active immunity occurs after infection; artificial active immunity comes from vaccines.
- Vaccines introduce weakened or dead pathogens, enabling the body to recognize and fight them in the future.
First Line of Defense (Non-Specific)
- The first line of defense is a physical and chemical barrier on body surfaces, not within tissues or blood.
- Examples include skin, tears (antimicrobial proteins), and mucous linings in respiratory tracks.
- Sweat creates an acidic environment on skin, deterring pathogen growth.
Second Line of Defense (Non-Specific)
- Activated if pathogens breach the first line; occurs in soft tissues, not blood.
- Inflammation involves swelling, heat, and fever as a general response to infection.
- Histamines induce swelling, increase blood flow, and make vessels permeable for white blood cells.
- Phagocytes (large white blood cells) engulf and digest pathogens via phagocytosis.
- Lysosomes in phagocytes break down pathogen components for removal.
Third Line of Defense (Specific)
- Involves targeted responses by T and B lymphocytes against specific pathogens in the bloodstream.
- Helper T cells (CD4) start immune responses and recruit B cells.
- Killer T cells destroy virus-infected or cancerous host cells by recognizing abnormal antigens.
- Suppressor T cells stop the immune response when infection is controlled.
B Lymphocytes and Antibody Production
- B cells recognize antigens and can become plasma cells or memory cells.
- Plasma cells produce antibodies, which bind to antigens and mark pathogens for destruction.
- Antibodies can burst bacteria, clump pathogens, enhance phagocyte activity, or neutralize toxins.
- Memory B cells enable faster response if the same pathogen infects again.
Key Terms & Definitions
- Immunity — Protection against infection by pathogens.
- Pathogen — Disease-causing organism (virus, bacteria, fungi, protist).
- Passive Immunity — Immunity from external antibodies, short-term.
- Active Immunity — Immunity from own immune response, long-term.
- Antigen — Protein on cell surfaces identifying “self” vs. “non-self.”
- Antibody — Y-shaped protein binding to specific antigens for pathogen removal.
- Phagocyte — White blood cell that engulfs and destroys pathogens.
- Histamine — Chemical causing inflammation and attracting immune cells.
- Plasma Cell — B cell variant producing large quantities of antibodies.
- Memory Cell — Lymphocyte remembering a pathogen for faster future response.
Action Items / Next Steps
- Review diagrams of the immune response pathways.
- Memorize the roles of key immune cells and types of immunity.
- Prepare for quiz on immunity terminology and processes.