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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 Biology of the Immune System).
Physical
Barriers
Usually, the skin prevents invasion by microorganisms unless it is damaged—for example, by 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 airways. 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 that drains urine from the body. In males older than 6 months, 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. 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 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, another type of white blood cell, increases in allergic reactions and many parasitic infections, but usually not in bacterial infections.
Certain infections, such as typhoid fever, actually lead to a decrease in the white blood cell count, but how these infections cause the decrease is not known.
Inflammation
Any injury, including an invasion by microorganisms, causes inflammation in the affected area. Inflammation, a complex reaction, results from many different conditions. Through release of different substances from the damaged tissue, inflammation directs the body's defenses to do the following:
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, causing 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 Biology of the Immune System: Acquired Immunity). 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 as a protective response to infection and injury. The elevated body temperature (fever) enhances the body's defense mechanisms, although it can cause discomfort. However, certain people (such as alcoholics, the very old, and the very young) may experience a drop in temperature in response to severe infection.
Temperature is considered elevated when it is higher than 100° F (37.8° C) as measured by an oral thermometer. Although 98.6° F (37° C) is considered normal temperature, body temperature varies throughout the day. It is lowest in the early morning and highest in the late afternoon—sometimes reaching 99.9° F (37.7° C).
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 does not stay at a constant temperature. Sometimes temperature peaks every day and then returns to normal. Alternatively, temperature varies but does not return to normal—called remittent fever. Doctors no longer think that the pattern of rise and fall of fever is very important in diagnosis.
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 and macrophages (another type of white blood cell). 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 may also 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, or a urinary tract infection). Usually, a doctor can easily diagnose the infection with a brief history, physical examination, and occasionally a few simple tests, such as a chest x-ray and urine tests. However, sometimes 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 produced by the body against its own tissues (autoimmune disorders), and 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 disorders, drugs currently being taken, exposure to infections, and recent travel. The pattern of the fever usually does not help with the diagnosis. However, there are 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 is also important. For example, people who work in a meatpacking plant are more likely to develop brucellosis.
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After asking questions, the doctor does 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. An increase in the white blood cell count usually indicates infection. The differential count (the proportion of different types of white blood cells) gives further clues. For example, an increase in neutrophils suggests a relatively new bacterial infection. An increase in eosinophils suggests the presence of parasites, such as tapeworms or roundworms.
A fever
of unknown origin may be diagnosed when people have a fever of at least 101° F (38.3° C) for several weeks and extensive investigation does not reveal a cause. In such cases, the cause may be an unusual chronic infection or something other than infection, such as a connective tissue disorder, cancer, or another disorder. 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 site of suspected infection. The specimen is then examined under a microscope and cultured.
Because fever helps the body defend against infection, and because fever itself is not dangerous (unless it is higher than about 106° F [41.1° C], there is some debate as to whether fever should be routinely treated. However, people with a high fever generally feel 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 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 Viral Infections: What Is Reye's Syndrome? ), which can be fatal.
Last full review/revision October 2008 by Allan R. Tunkel, MD, PhD
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