If the body did not have defenses against infection, it would quickly be overwhelmed by microorganisms. These defenses require a living, properly functioning body. A dead body begins to decay almost immediately because its defenses are no longer working.
Natural barriers and the immune system Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more defend the body against organisms that can cause infection Types of Infectious Organisms . (See also Lines of Defense Lines of Defense The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more .)
Natural barriers include the skin, mucous membranes, tears, earwax, mucus, and stomach acid. Also, the normal flow of urine washes out microorganisms that enter the urinary tract.
The immune system uses white blood cells White blood cells The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more and antibodies Antibodies One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more to identify and eliminate organisms that get through the body’s natural barriers.
(See also Overview of Infectious Disease Overview of Infectious Disease Microorganisms are tiny living creatures, such as bacteria and viruses. Microorganisms are present everywhere. Despite their overwhelming abundance, relatively few of the thousands of species... read more .)
Natural Barriers Against Infection
Usually, the skin Structure and Function of the Skin The skin is the body’s largest organ. It serves many important functions, including Protecting the body against trauma Regulating body temperature Maintaining water and electrolyte balance Sensing... read more prevents invasion by microorganisms unless it is damaged (for example, by an injury, insect bite, or burn).
Mucous membranes, such as the lining of the mouth, nose, and eyelids, are also effective barriers. Typically, mucous membranes are coated with secretions that fight microorganisms. For example, the mucous membranes of the eyes are bathed in tears, which contain an enzyme called lysozyme that attacks bacteria and helps protect the eyes from infection.
The airways Overview of the Respiratory System To sustain life, the body must produce sufficient energy. Energy is produced by burning molecules in food, which is done by the process of oxidation (whereby food molecules are combined with... read more filter out particles that are present in the air that is inhaled. The walls of the passages in the nose and airways are coated with mucus. Microorganisms in the air become stuck to the mucus, which is coughed up or blown out of the nose. Mucus removal is aided by the coordinated beating of tiny hairlike projections (cilia) that line the airways. The cilia sweep the mucus up the airways, away from the lungs.
The digestive tract Overview of the Digestive System The digestive system, which extends from the mouth to the anus, is responsible for receiving food, breaking it down into nutrients (a process called digestion), absorbing the nutrients into... read more has a series of effective barriers, including stomach acid Stomach The stomach is a large, bean-shaped, hollow muscular organ consisting of four regions: Cardia Fundus Body Antrum read more , pancreatic enzymes Pancreas The pancreas is an organ that contains two types of glandular tissue: Pancreatic acini Islets of Langerhans (See also Overview of the Digestive System.) The acini produce digestive enzymes.... read more , bile Gallbladder and Biliary Tract The gallbladder is a small, pear-shaped, muscular storage sac that holds bile and is interconnected to the liver by ducts known as the biliary tract. (See also Overview of the Liver and Gallbladder... read more , and intestinal secretions. These substances can kill bacteria or prevent them from multiplying. The contractions of the intestine (peristalsis, which moves contents of the bowel through the digestive tract), and the normal shedding of cells lining the intestine help remove harmful microorganisms.
The urinary tract Overview of the Urinary Tract Normally, a person has two kidneys. The rest of the urinary tract consists of the following: Two ureters (the tubes connecting each kidney to the bladder) The bladder (an expandable muscular... read more also has several effective barriers. The bladder is protected by the urethra Urethra The urethra is a tube that drains urine from the bladder out of the body. In men, the urethra is about 8 inches (20 centimeters) long, ending at the tip of the penis. In women, the urethra is... read more , the tube that drains urine from the body. In males, the urethra is long enough that bacteria are seldom able to pass through it to reach the bladder, unless the bacteria are unintentionally placed there by catheters or surgical instruments. In females, the urethra is shorter, occasionally allowing external bacteria to pass into the bladder. In both sexes, when the bladder empties, it flushes out any bacteria that reach it.
The vagina Vagina The internal genital organs form a pathway (the genital tract). This pathway consists of the following: Vagina (part of the birth canal), where sperm are deposited and from which a baby can... read more is normally acidic. The acidity of the vagina Causes Bacterial vaginosis is a common condition that occurs when the balance of bacteria in the vagina (vaginal microbiome) is altered. Bacterial vaginosis causes a thin, yellow-green or gray discharge... read more prevents harmful bacteria from growing and helps maintain the number of protective bacteria.
The body also defends against infection by increasing the number of certain types of white blood cells (neutrophils Neutrophils One of the body's lines of defense (immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more and monocytes Monocytes and Macrophages One of the body's lines of defense (immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more ), which engulf and destroy invading microorganisms. The increase can occur within several hours, largely because white blood cells are released from the bone marrow, where they are made. The number of neutrophils increases first. If an infection persists, the number of monocytes increases. The blood carries white blood cells to sites of infection.
The number of eosinophils Eosinophils One of the body's lines of defense (immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more , another type of white blood cell, increases in allergic reactions and many parasitic infections, but usually not in bacterial infections.
However, certain infections, such as typhoid fever Typhoid Fever Typhoid fever is caused by certain types of the gram-negative bacteria Salmonella. It typically causes a high fever and abdominal pain. Typhoid fever can be spread by consuming food or... read more , viral infections, and bacterial infections that overwhelm the immune system, can lead to a decrease in the white blood cell count.
Any injury, including an invasion by microorganisms, causes inflammation in the affected area. Inflammation, a complex reaction, results from many different conditions. The damaged tissue releases substances that cause inflammation and that direct the immune system to do the following:
Wall off the area
Attack and kill any invaders
Dispose of dead and damaged tissue
Begin the process of repair
However, inflammation may not be able to overcome large numbers of microorganisms.
During inflammation, the blood supply increases, helping carry immune cells to the affected area. Because of the increased blood flow, an infected area near the surface of the body becomes red and warm. The walls of blood vessels become more porous, allowing fluid and white blood cells to pass into the affected tissue. The increase in fluid causes the inflamed tissue to swell. The white blood cells attack the invading microorganisms and release substances that continue the process of inflammation.
Other substances trigger clotting in the tiny vessels (capillaries) in the inflamed area, which delays the spread of the infecting microorganisms and their toxins.
Many of the substances produced during inflammation stimulate the nerves, causing pain. Reactions to the substances released during inflammation include the chills, fever, and muscle aches that commonly accompany infection.
When an infection develops, the immune system Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more also responds by producing several substances and agents that are designed to attack the specific invading microorganisms ( see Acquired Immunity Acquired Immunity One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more ). Examples are
Killer T cells T cells (a type of white blood cell) that can recognize and kill the invading microorganism
Antibodies attach to and immobilize microorganisms. They kill them outright or help neutrophils target and kill them.
How well the immune system defends the body against each microorganism depends partly on a person's genetic make-up.
Body temperature increases as a protective response to infection and injury. An elevated body temperature (fever Fever in Adults Fever is an elevated body temperature that occurs when the body's thermostat (located in the hypothalamus in the brain) resets at a higher temperature, primarily in response to an infection... read more ) enhances the body’s defense mechanisms, although it can cause discomfort.
A part of the brain called the hypothalamus controls body temperature. Fever results from an actual resetting of the hypothalamus's thermostat. The body raises its temperature to a higher level by moving (shunting) blood from the skin surface to the interior of the body, thus reducing heat loss. Shivering (chills) may occur to increase heat production through muscle contraction. The body's efforts to conserve and produce heat continue until blood reaches the hypothalamus at the new, higher temperature. The new, higher temperature is then maintained. Later, when the thermostat is reset to its normal level, the body eliminates excess heat through sweating and shunting of blood to the skin.
Certain people (such as the very old, the very young, and people with an alcohol use disorder) are less able to generate a fever. These people may experience a drop in temperature in response to severe infection.