The Body's Defenses Against Infection
Physical barriers and the immune system defend the body against organisms that can cause infection. Physical 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 and antibodies to identify and eliminate organisms that get through the body's physical barriers (see Section 16, Chapter 183).
Physical Barriers
Usually, the skin prevents invasion by microorganisms unless it is damaged--for example, from an injury, insect bite, or burn. Other effective physical barriers are the mucous membranes, such as the linings of the mouth, nose, and eyelids. 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 filter out particles that are present in the air that is breathed in. 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 air passages. The cilia sweep the mucus up the airways, away from the lungs.
The digestive tract has a series of effective barriers, including stomach acid, pancreatic enzymes, bile, and intestinal secretions. The contractions of the intestine (peristalsis) and the normal shedding of cells lining the intestine help remove harmful microorganisms.
The bladder is protected by the urethra, the tube through which urine passes as it leaves the body. In males older than 6 months of age, 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 for passage of external bacteria into the bladder. The flushing effect as the bladder empties is another defense mechanism in both sexes. The vagina is protected by its normal acidic environment.
The Blood
One way the body defends against infection is by increasing the number of certain types of white blood cells (neutrophils and monocytes), which engulf and destroy invading microorganisms. The increase can occur within several hours, largely because of the release of white blood cells from the bone marrow. The number of neutrophils increases first. If an infection persists, the number of monocytes increases. The number of eosinophils, another type of white blood cell, increases in allergic reactions and many parasitic infections, but usually not with bacterial infections.
Certain infections, such as typhoid fever, actually lead to a decrease in the white blood cell count, but how these infections do this is not known.
Inflammation
Any injury, including an invasion by microorganisms, causes a complex reaction called inflammation in the affected area. Inflammation occurs as a result of many different conditions. Through the release of different substances from the damaged tissue, inflammation directs the body's defenses to wall off the area, attack and kill any invaders, dispose of dead and damaged tissue, and begin the process of repair. However, inflammation may not be able to overcome large numbers of microorganisms.
During inflammation, the blood supply increases. 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, producing pain. Reactions to the substances released during inflammation include the chills, fever, and muscle aches that commonly accompany infection.
Immune Response
When an infection develops, the immune system responds by producing several substances and agents that are designed to attack the specific invading microorganisms (see Section 16, Chapter 183). For example, the immune system may create killer T cells (a type of white blood cell) that can recognize and kill the invading microorganism. Also, the immune system produces antibodies that are specific to the invading microorganism. Antibodies attach to and immobilize microorganisms--killing them outright or helping the neutrophils target and kill them.
Fever
Body temperature increases (fever) as a protective response to infection and injury. The elevated body temperature enhances the body's defense mechanisms, although it can cause discomfort for the person. Temperature is considered elevated when it is higher than 100° F as measured by an oral thermometer. Although 98.6° F is considered "normal" temperature, body temperature varies throughout the day, being lowest in the early morning and highest in the late afternoon--sometimes reaching 99.9° F.
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.
Fever may follow a pattern: sometimes temperature peaks every day and then returns to normal. Alternatively, fever may be remittent, in which the temperature varies but does not return to normal. Certain people (for example, alcoholics, the very old, and the very young) may experience a drop in temperature as a response to severe infection.
Substances that cause fever are called pyrogens. Pyrogens can come from inside or outside the body. Microorganisms and the substances they produce (such as toxins) are examples of pyrogens formed outside the body. Pyrogens formed inside the body are usually produced by monocytes. Pyrogens from outside the body cause fever by stimulating the body to release its own pyrogens. However, infection is not the sole cause of fever; fever also may result from inflammation, cancer, or an allergic reaction.
Usually, fever has an obvious cause, which is often--but not always--an infection (such as influenza, pneumonia, a urinary tract infection, or some other infection) that a doctor can easily diagnose with a brief history, physical examination, and occasionally a few simple tests, such as a chest x-ray and urine tests. Sometimes, however, the cause is not readily discernible.
If fever continues for several days and has no obvious cause, a more detailed investigation is required. There are many potential causes of such a fever; common causes in adults include infections, diseases caused by antibodies against the person's own tissues (autoimmune diseases), and an undetected cancer (especially leukemia or lymphoma).
To determine the cause of a fever, a doctor begins by asking a person about present and previous symptoms and diseases, drugs currently being taken, exposure to infections, and recent travel. The pattern of the fever usually does not help with the diagnosis. There are, however, some exceptions: a fever that recurs every other day or every third day is typical of malaria.
Recent travel (especially overseas) may give clues to the cause of a fever, because some infections occur only in certain areas. For example, coccidioidomycosis (a fungal infection) occurs almost exclusively in the southwestern United States. A history of exposure to certain materials or animals also is important. For example, a person who works in a meatpacking plant is more likely to develop brucellosis.
After asking questions, the doctor performs a thorough physical examination to find a source of infection or evidence of disease. Blood and other body fluids may be sent to the laboratory to try to grow the microorganism in a culture. Other blood tests can be used to detect antibodies against specific microorganisms. Increases in the white blood cell count usually indicate infection. The differential count (the proportion of different types of white blood cells) gives further clues. An increase in neutrophils, for example, suggests an acute bacterial infection. An increase in eosinophils suggests the presence of parasites--for example, tapeworms or roundworms.
When a person has a fever of at least 101° F for several weeks and extensive investigation fails to reveal a cause, a doctor may refer to it as a fever of unknown origin. In such cases, the cause may be an unusual chronic infection or something other than infection, such as a connective tissue disease, cancer, or some other disease. Ultrasonography, computed tomography (CT), or magnetic resonance imaging (MRI) may help a doctor diagnose the cause. Injection of white blood cells labeled with a radioactive marker can be used to identify areas of infection or inflammation. If these test results are negative, the doctor may need to obtain a biopsy specimen from the liver, bone marrow, or another suspected site. The specimen is then examined under a microscope and cultured.
Because fever helps the body defend against infection, there is some debate as to whether it should be routinely treated. However, a person with a high fever generally feels much better when the fever is treated.
Drugs used to lower body temperature are called antipyretics. The most effective and widely used antipyretics are acetaminophen and the nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin and ibuprofen. However, aspirin should not be given to children and teenagers to treat a fever because it increases the risk of Reye's syndrome (see Section 23, Chapter 273), which can be fatal.
 See the sidebar Some Causes of Fever.
|
