Nonspecific External and Internal Defenses
Nonspecific External Defenses The skin and mucous membranes form nonspecific external barriers to invasion In our bodies, the first line of defense consists of the surfaces with direct exposure to the environment The skin The outer surface of the skin consists of dry, dead cells filled with tough proteins that do not allow the microbes to obtain the water and nutrients they need to survive The secretions from sweat and sebaceous glands contain natural antibiotics, such as lactic acid, that inhibit the growth of many bacteria and fungi
Nonspecific External Defenses The skin and mucous membranes form nonspecific external barriers to invasion In our bodies, the first line of defense consists of the surfaces with direct exposure to the environment The mucous membranes of the digestive, respiratory, and urogenital tracts secrete mucus, which traps microbes that enter the nose or mouth Mucus contains proteins, including lysozome, which kills bacteria by digesting their cell walls, and defensin, which makes holes in bacterial plasma membranes cilia on the membranes sweep up the mucus, microbes and all, until it is either coughed or sneezed out of the body or is swallowed Bacteria trapped by mucus
Nonspecific External Defenses The skin and mucous membranes form nonspecific external barriers to invasion Mucus, antibacterial proteins, and ciliary action defend the mucous membranes against microbes The slight acidity of urine inhibits bacterial growth In females, acidic secretions and mucus help protect the vagina Fluids released by the body, including tears, urine, diarrhea, and vomit, help expel invaders
Nonspecific Internal Defenses The INNATE IMMUNE response nonspecifically combats invading microbes Despite the many defenses, many disease-causing microbes enter the body through the mucous membranes or through cuts in the skin Pathogens that get through the external barriers encounter three types of nonspecific innate immune responses Protection by white blood cells The body has a standing army of white blood cells, or leukocytes, many of which are specialized to attack and destroy invading cells or the body’s own cells if they have been infected by viruses The inflammatory response A wound provokes an inflammatory response, which recruits leukocytes to the site of injury and walls off the injured area, isolating the infected tissue from the rest of the body Fever If a population of microbes succeeds in establishing a major infection, the body may produce a fever, which slows down microbial reproduction and enhances the body’s own fighting abilities
Nonspecific Internal Defenses Phagocytic leukocytes and natural killer cells destroy invading microbes The body has several types of leukocytes, collectively known as phagocytes, which ingest foreign invaders and cellular debris by phagocytosis Three important types of phagocytes are Macrophages Neutrophils Dendritic cells Bacteria visible through a hole in the macrophage’s plasma membrane A macrophage stuffed with bacteria that it has ingested A macrophage leaves a capillary and enters a wound
Nonspecific Internal Defenses The inflammatory response attracts phagocytes to injured or infected tissue The inflammatory response causes tissues to become warm, red, swollen, and painful The inflammatory response begins when damaged cells release chemicals that cause certain cells in the connective tissue, called mast cells, to release histamine Extra blood flowing through leaky capillaries drives fluid from the blood and into the wounded area, causing redness, warmth, and swelling Swelling and some of the chemicals released by the injured tissue cause pain, which usually leads to protective behaviors that reduce the likelihood of further injury
Tissue damage carries bacteria into the wound dead cell layer Tissue damage carries bacteria into the wound epidermis Wounded cells release chemicals (red) that stimulate mast cells Mast cells release histamine (blue) dermis Histamine increases capillary blood flow and permeability Phagocytes leave the capillaries and ingest bacteria and dead cells 8
Nonspecific Internal Defense Fever combats large-scale infections If invaders breach these defenses and mount a full-blown infection, they may trigger a fever The onset of fever is controlled by the hypothalamus, the part of the brain housing temperature- sensing nerve cells that serve as the body’s thermostat In humans, the thermostat is set at about 97 to 99F Certain types of bacteria, as well as the phagocytic cells that respond to an infection, produce chemicals called pyrogens Pyrogens travel in the bloodstream to the hypothalamus and raise the thermostat’s set point Fever causes the cells of the adaptive immune system to multiply more rapidly, hastening the onset of an effective adaptive immune response