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What is the function of fever in disease? When researcher Matthew Kluger infected a group of desert lizards with bacteria, he
found that raising their body temperature to achieve a fever improved their chances of survival.
Because reptiles are cold-blooded, Kluger used sunlamps to raise the lizards' body heat,
speeding up their bodies' defenses. Did fever evolve in part to help organisms survive infection?
Credits: From "La Fièvre" by Matthew J. Kluger, La Recherche 12: 688-696, 1981.
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Function of Fever:
When a child or parent becomes feverish with shivers, chills, and sweats, our first thought is to get the temperature down.
Pharmacies sell billions of fever-reducing pills like aspirin and acetaminophen every year, and schools often insist that
students stay home until their fever is gone. But is this "fever phobia" backed up by science?
Increasingly, medical researchers are discovering that fever has endured in mammals and other creatures for good reasons,
though the reasons why are not clear. Often, a fever in response to an infection is actually a reflection of the body's defenses
going into high gear. Some parts of the immune system work better at a higher temperature, which strengthens resistance to
infection and increases the odds of survival.
The new thinking is that mild fever can be a positive adaptation and shouldn't necessarily be treated. At other times, though,
fever may spur the microbes' growth rate by raising the temperature of the host body. In this case, the attackers have
evolved a way to chemically manipulate the host's immune system for their own advantage. And a high fever is a danger sign,
especially in young children.
What is this mysterious phenomenon, fever? It's not simply a rise in body temperature. It is an upward shift in the body's "set
point," or core temperature, which is regulated by the hypothalamus in the brain. In response to an infection, the body
releases chemicals that cause a sensation of being cold. The hypothalamus then raises the set point by making the body burn
fat, reduce blood flow to the skin, and shiver. Most of the time, fever isn't dangerous in itself, but a patient will feel more
comfortable at a lower temperature.
In a dramatic demonstration of fever's benefits, researcher Matthew Kluger infected desert iguanas with bacteria. Because
these lizards are cold-blooded, they could only warm their bodies by seeking outside heat -- in this case, sunlamps. All except
one of 13 iguanas sought the warmth to raise their temperatures, and those 12 survived; the other one died. After that,
Kluger injected 12 other iguanas with live bacteria, and also gave them a fever-fighting drug. Five of them failed to develop
a fever, and died as a result. The other seven, which somehow became feverish despite the drug, survived.
Despite experiments like this, scientists haven't yet answered all their questions about this common and ancient body
symptom.
Homeostasis
Certainly you've noticed that on a hot day, especially when you're active, your skin perspires. This is your body's way of
cooling itself. And when you run down the street to catch a bus, your body responds by increasing your heart rate and your
breathing rate to supply your hardworking muscle cells with more oxygen. These bodily adjustments happen automatically,
without your having to think about them. But if you're not at the controls, making sure your body gets enough oxygen and
nutrients and doesn't overheat, then who or what is?
Biologists divide the nervous system into two parts: the somatic and the autonomic. The somatic system is responsible for all
voluntary actions, like brushing your teeth or throwing a ball. The autonomic system handles all involuntary actions, including
breathing, digesting, sweating, and the rhythmic beating of your heart.
The autonomic system can be broken down into two divisions: the parasympathetic system and the sympathetic system. The
center of the parasympathetic system is a part of the brain near the top of the spinal cord called the medulla. The medulla
communicates with various organs and glands through a large nerve called the vagus nerve and its many branches. The center
of the sympathetic system is the middle portion of the spinal cord, in the thoracic and lumbar regions.
Although most organs are connected to both the parasympathetic and sympathetic systems, they do not receive the same
type of information from the two systems. In fact, more often than not the two systems send opposite signals, though usually
not at the same time. An organ that is stimulated to perform some function by the parasympathetic system will be inhibited
to perform that function by the sympathetic system, and vice versa. For example, the parasympathetic system stimulates the
stomach to secrete digestive juices, while the sympathetic inhibits such secretion; the sympathetic system causes arteries to
dilate and the parasympathetic causes them to constrict. In general, the parasympathetic system stimulates organs that have
relatively tranquil functions, including digestion, while the sympathetic system stimulates organs that have a more active
function, especially those involved in "fight or flight" responses.
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Homeostasis and the Immune System
The immune system is a collection of molecules, cells, and organs whose complex interactions form a defense network
capable of protecting the body from outside invaders. Immune cells, called lymphocytes, are critical to the proper
functioning of the immune system. When they encounter foreign cells, some lymphocytes respond by producing antibodies -large proteins that destroy or otherwise interfere with the vital activities of foreign cells. Other types of lymphocytes, called
T cells, either actively kill the foreign cells using powerful chemicals, or secrete chemicals that attract macrophages that eat
foreign cells.
A healthy immune system can successfully fight invaders such as the virus that causes mumps. Viruses like the human
immunodeficiency virus (HIV), the virus that causes AIDS, however, are not so easily dealt with. This is because the virus
attacks the T cells themselves, disrupting the body's natural immune response. To combat the virus's onslaught, the immune
system produces a billion new T cells every day. Unfortunately, the viruses also replicate about a billion times each day.
In a person infected with HIV, the body's immune system fights the virus -- day after day, month after month, sometimes year
after year. Eventually, though, the immune system exhausts itself. The number of T cells drops dramatically, and without
this vital component of the immune system, the body is left susceptible to other diseases.
As has happened with some other diseases caused by microbes, many medical researchers believe that AIDS may one day be
cured by a vaccine. Vaccines work by introducing the body to otherwise harmful microorganisms in a safe, controlled
manner. Often the microbes in a vaccine are dead or otherwise rendered harmless. Even dead viruses, however, are often
enough to elicit the body's immune response -- its ability to recognize and fight off a particular virus should it ever come into
contact with the virus again.
In many ways the creation of a vaccine seems like an obvious approach for curing AIDS. It worked surprisingly well in the
past. Polio used to kill or maim more than a million people worldwide every year. Today, this disease is but a distant
memory. Unfortunately, the vaccine approach worked as well as it did with diseases like because the human immune system
already had some innate ability to fight the viruses that caused them. Generation after generation of human populations had
evolved with these viruses and, thus, had become better at fighting them and the diseases they cause. Indeed, humans have
at least some natural ability to fight most viruses. That, unfortunately, is not the case with HIV, perhaps because human
beings have not been exposed to the virus for long enough.
Recent findings suggest that HIV has been in the human population for only a short time. The virus, according to at least one
study, probably jumped from chimpanzees to humans very recently -- maybe only a few decades ago. If this is the case,
researchers surmise that our species would not have had time to evolve resistance to the virus's effects, as it seems
chimpanzees and a few other species with viruses similar to HIV have. Despite the setback in what some thought would be a
simple solution to a global epidemic, many researchers remain committed to the idea of an AIDS vaccine. Toward this end,
some are studying a small group of people -- perhaps 5 percent of the HIV-positive population -- who have the virus but don't
show any symptoms and whose infection remains at very low levels. Many experts believe these people's immune systems
have learned to fight the virus differently than other people's -- in a way that the body could "learn" through a vaccine.