1. First Line of Defense: Physical and Chemical Barriers
* Skin: The largest organ in the body, the skin acts as a tough barrier that prevents entry of microbes.
* Mucous membranes: These line the respiratory, digestive, and urinary tracts, trapping pathogens in mucus and moving them out with cilia.
* Body fluids: Tears, saliva, and other fluids contain lysozyme, an enzyme that breaks down bacterial cell walls.
* Stomach acid: The highly acidic environment of the stomach kills many ingested microbes.
* Normal flora: Beneficial bacteria that reside in the gut, skin, and other areas compete with harmful bacteria for resources and space.
2. Second Line of Defense: Innate Immune System
* Phagocytosis: White blood cells like neutrophils and macrophages engulf and destroy pathogens.
* Inflammation: A localized response to infection characterized by redness, swelling, heat, and pain. This helps isolate and destroy pathogens.
* Fever: An elevated body temperature, which inhibits the growth of many pathogens and increases immune responses.
* Antimicrobial proteins: Complement proteins and interferons directly attack pathogens or enhance other immune responses.
* Natural Killer (NK) cells: These lymphocytes destroy infected cells and cancer cells.
3. Third Line of Defense: Adaptive Immune System
* Lymphocytes: T cells and B cells recognize and attack specific pathogens.
* Antibody-mediated immunity: B cells produce antibodies that bind to and neutralize pathogens.
* Cell-mediated immunity: T cells directly kill infected cells or activate other immune cells.
* Memory cells: These long-lived lymphocytes provide lasting immunity to specific pathogens.
Key Concepts:
* Specificity: The adaptive immune system can recognize and target specific pathogens.
* Memory: The adaptive immune system can remember past infections and mount a faster and stronger response upon re-exposure.
* Self-tolerance: The immune system is able to differentiate between "self" cells and "non-self" pathogens, preventing attacks on the body's own tissues.
Interplay of the Lines of Defense:
While these lines of defense are often described separately, they actually work in a coordinated manner. The first line of defense prevents pathogens from entering the body. If they do, the second line of defense kicks in. The third line of defense is slower to respond but provides a highly specific and long-lasting protection.
Beyond the Lines of Defense:
* Vaccines: Introduce weakened or inactivated pathogens to stimulate the adaptive immune system and create immunity without causing disease.
* Antibiotics: Target specific bacterial infections and can help combat disease.
* Antiviral medications: Can help control viral infections.
Understanding these lines of defense is crucial for appreciating the complexity and effectiveness of the immune system, and for developing strategies to prevent and treat infections.