Summary
Highlights
The immune system is a complex network of tissues, organs, and specialized cells that protect the body from constant threats like bacteria and viruses. It comprises two main defense systems: the innate (nonspecific) system, which provides immediate, general protection through physical barriers, chemical defenses, and specialized cells; and the adaptive (specific) system, which targets specific pathogens and remembers past invaders. This video focuses on the innate immune system.
The body's primary innate defense is its physical barriers. Skin acts as a tough, keratinized epithelial membrane that prevents microorganisms from entering. Mucous membranes line body cavities exposed to the outside world (respiratory, digestive, urinary, reproductive tracts) and provide a similar barrier. These barriers also employ chemical weapons: stomach acid kills pathogens in food, sticky nasal mucus traps viruses, saliva and lacrimal fluid contain bacteria-fighting enzymes, and defensins in skin and membranes prevent bacterial and fungal growth in inflamed areas.
When the initial physical barriers are breached, the body activates its second line of internal innate defenses. Phagocytes, such as neutrophils and macrophages, are cells that engulf and digest pathogens. Neutrophils are abundant white blood cells that self-destruct after consuming a pathogen, forming pus. Macrophages, derived from monocytes, are larger, tougher phagocytes that can continuously consume invaders. Additionally, natural killer (NK) cells patrol the blood and lymph, identifying and destroying abnormal body cells, such as those infected with viruses or cancerous cells, by inducing apoptosis (programmed cell death) when they detect the absence of the MHC1 protein.
Injury triggers an inflammatory response, an internal alarm system that uses chemical signals to contain pathogens, clean the area, and promote healing. This response results in redness, swelling, heat, and pain. Mast cells release histamine, causing vasodilation (increased blood flow, leading to redness and heat, which boost metabolic rates for faster healing) and increased blood vessel permeability (leading to swelling, which helps clot blood and filter fluid). This also attracts phagocytes and lymphocytes to destroy pathogens and remove dead cells. Neutrophils are released from bone marrow and guided to the injury site by chemical signals, squeezing through vessel walls to fight infection. Monocytes then transform into macrophages to continue the clean-up.
If local defenses are overwhelmed, white blood cells and macrophages release pyrogen chemicals, which raise the body's temperature, inducing a fever. This systemic fever increases cellular metabolism for faster healing and instructs the liver and spleen to sequester iron and zinc, inhibiting bacterial growth. While potent, sometimes even these generalized innate defenses are not enough, necessitating the specialized response of the adaptive immune system.