CHAPTER 40 – THE IMMUNE
SYSTEM AND DISEASE
40-1 Infectious Disease
• A disease is any change, other than an injury,
that disrupts the normal functions of the body.
• Some diseases, such as hemophilia, are
inherited. Others are caused by materials in the
environment, such as cigarette smoke. Some
others are produced by things such as bacteria,
viruses, or pathogens.
• The Germ Theory of Disease
• The germ theory of disease states that infectious
diseases are caused by microorganisms, or
germs. This theory came about through the
works of a French chemist named Louis Pasteur
and a German bacteriologist named Robert
Koch. Prior to this theory, people thought
diseases were brought on by curses or evil
spirits.
• Koch’s Postulates
• Koch developed a series of guidelines still used today to identify the
microorganism that causes a specific disease. Koch’s postulates state
the following:
• 1. The pathogen should always be found in the body of a sick
organism and should not be found in a healthy one.
• 2. The pathogen must be isolated and grown in the laboratory in pure
culture
• 3. When the purified pathogens are placed in a new host, they
should cause the same disease that infected the original host.
• 4. The injected pathogen should be re-isolated from the second host.
It should be identical to the original pathogen.
This made us realize that if a particular pathogen
could be identified, maybe the disease it caused
could be prevented or cured.
• Toxins – poisons that produce illness by parasites
that live and feed inside an infected organism.
• Some remove nutrients from the digestive
system. Others destroy blood cells and neurons.
A virus, which is a nonliving pathogen, can use
the materials of a host cell to make copies of the
virus until the cell bursts.
How Diseases are Spread
• The best method for fighting any disease is to
avoid it.
• Infectious diseases are transmitted in a number
of ways. Some infectious diseases are spread
from one person to another through coughing,
sneezing, or physical contact. Other infectious
diseases are spread through contaminated water
or food. Others are spread by infected animals.
Animals that carry disease-causing organisms
from person to person are called vectors. (ex.
Ticks or mosquitoes). It is important to try to
avoid contact with vectors. Some ways are
avoiding tall grass and wooded areas and spraying
bug spray.
The common cold, mumps, measles, and the flu spread
form one person to another through coughing,
sneezing, or hand-to-hand contact. To avoid the
spread, it is important to cover your mouth with a
tissue when you cough or sneeze and washing your
hands thoroughly.
• Sexually transmitted diseases (STDs) are spread
through sexual contact. Syphilis and gonorrhea
are caused by bacteria. Others, like hepatitis B,
hepatitis C, genital herpes, and AIDS are caused
by viruses.
• The consequences of having an STD can be
severe. Gonorrhea and chlamydia can
permanently damage the reproductive system.
Syphilis and AIDS can be fatal.
Fighting Infectious Diseases
• If a disease was not able to be prevented, drugs
have been developed for use against all sorts of
pathogens.
• Antibiotics are compounds that kill bacteria
without harming the cells of humans or animals.
Antibiotics interfere with the cellular processes
of microorganisms.
• Antibiotics have no effect on viruses, only
bacteria.
40-2 The Immune System
• Skin, mucus, sweat, and tears help to keep
pathogens out. Your body’s most important
nonspecific defense is the skin. Very few
pathogens can get through the skin’s surface. Oil
and sweat glands kill many types of bacteria.
• When skin is broken, pathogens can enter your
body and multiply. This can cause an infection,
which causes swelling, redness, and pain.
Pathogens can also enter your body through your
mouth and nose. Mucus in your nose and throat
traps viruses and bacteria and cilia push them
away from your lungs. Stomach acid and
digestive enzymes destroy many pathogens that
make their way to your stomach. Mucus, saliva,
sweat, and tears contain lysozyme, which is an
enzyme that breaks down the cell walls of many
bacteria.
If pathogens do manage to enter your body, they
may multiply quickly, releasing toxins into your
tissues. When this happens, the inflammatory
response, which is a second line of defense, is
activated. The inflammatory response is a
nonspecific defense reaction to tissue damage
caused by injury or infection. Blood vessels near
the wound expand and white blood cells leak
from the vessels to enter the infected tissues.
Many of these white blood cells are phagocytes,
which engulf and destroy bacteria.
• When pathogens are detected, the immune
system produces millions of white blood cells,
which fight the infection. The immune system
also releases chemicals that increase the core
body temperature. This is a fever.
• Interferons are a group of proteins produced by
virus-infected cells that help other cells resist
viral infection.
Specific Defenses
• If a pathogen is able to get past the body’s nonspecific defenses, the immune system reacts
with a series of specific defenses that attack the
disease-causing agent. These defenses are
called immune response.
• Antigen- a substance that triggers this response.
• The body can produce two different immune
responses: humoral immunity and cell-mediated
immunity.
• Humoral immunity – immunity against
pathogens in the body fluids (blood and lymph).
It is produced by lymphocytes (a type of white
blood cell).
• An antibody molecule is the basic functional unit
of the humoral immune response.
• Antibody – a protein that helps destroy
pathogens
• Cell-Mediated Immunity
• Cell-mediated immunity relies on lymphocytes,
not antibodies. Helper T cells regulate the
production of antibodies by B cells. Other T cells
can attack antigen-carrying cells directly. The
most effective attacking cells in the immune
system are killer T cells. These transfer proteins
into the cell membrane of a pathogen, causing
fluid from inside the cell to leak out of the cell to
rupture and die.
• Killer T cells make it difficult to do organ
transplants. The immune system recognizes
cells. When an organ from one person is
transplanted into another, the second person’s
immune system recognizes the transplanted
organ as foreign and attacks it. This is known as
rejection. To prevent this, doctors look for a
donor whose markings are nearly identical to the
markers of the recipient. Recipients must take
drugs that suppress the cell-mediated immune
response.
• People can acquire permanent immunity to
certain diseases. Once the body has been
exposed to a pathogen, millions of memory B
and T cells remain capable of producing specific
antibodies to that pathogen. Memory B cells
make sure the disease never gets a chance to
develop a second time.
• Vaccination- the injection of a weakened or mild
form of a pathogen to produce immunity.
• Active immunity is produced because the body
of the recipient has the ability to produce an
immediate active immune response against the
pathogen.
40-3 Immune System Disorders
There are two main types of disorders. In the
first, the immune system may overreact to an
antigen, producing discomfort or even disease. In
the second, the cellular nature of the immune
response is a potential weak point.
• Allergies – result when antigens from allergens
bind to mast cells (a type of immune cell)
• When allergy-causing antigens attach
themselves to mast cells, the activated mast cells
release chemicals called histamines. Histamines
produce sneezing, runny eyes and nose, and
other irritations that make a person with
allergies so uncomfortable.
• Some allergens include ragweed pollen, dust,
and dust mites.
• Allergic reactions can cause Asthma.
• Asthma is when the smooth muscle contractions
reduce the size of air passageways in the lungs
and make breathing very difficult.
• A particular antigen usually triggers an asthma
attack.
Autoimmune Disease
• When the immune system makes a mistake and
attacks the body’s own cells, it produces an
autoimmune disease.
• Examples: juvenile-onset diabetes and multiple
sclerosis
AIDS
• AIDS (Acquired Immune Deficiency Syndrome) is
caused by a virus. The virus is known as HIV (human
immunodeficiency virus).
• Once HIV enters the body, it attaches to receptors on
the surfaces of the T cells that help other lymphocytes
respond to infection. The body does produce
antibodies against HIV, but HIV replicates within cells
of the immune system, so it is less likely to bind to
antibodies. Gradually, HIV kills off most of the helper
T cells.
40-4 Cancer
Cancer is a life-threatening disease in which cells
multiply uncontrollably and destroy healthy
tissue.
• Cancers begin when something goes wrong with
the controls that normally regulate cell growth
and division.
• Tumor – a mass of growing tissue, resulting from
a single cell or a group of cells that begins to
grow and divide uncontrollably.
• Cancerous tumors are malignant, which means
they can invade and destroy surrounding healthy
tissue.
• Metastasis- the spread of cancerous tumors
beyond their original site.
Causes of Cancer
• Cancer may be inherited, may be caused by
viruses (ex – HPV), or may result from mutations
in DNA.
Fighting Cancer
• Treatments for cancer fall into three general
categories: surgery, radiation therapy, and drug
therapy (chemotherapy).