Infectious Diseases and the Immune System
The human body is continuously exposed to pathogens, such as
viruses and bacteria. When one of these pathogens enters the
body and begins to multiply, it causes an infectious disease.
This section examines the body’s nonspecific defenses, which
are the first lines of protection against invading pathogens.
1. Immunity may be defined as the body’s ability to destroy
pathogens or other foreign material and to prevent further cases
of certain infectious diseases. This ability is of vital
importance because the body is exposed to pathogens from the
moment of birth.
WHAT IS DISEASE?
1. Any change, other than an injury, that interferes with
normal functioning of the body is a disease.
2. Different diseases can be recognized by their symptoms,
or changes they produce in the body.
3. Many different things can cause diseases - infectious
diseases are produced by pathogens. Pathogens are
disease-causing microorganisms, such as viruses, bacteria,
fungi, and protozoans.
4. When the body is successfully invaded by a pathogen, we
say that an infection has occurred.
5. The numbers of microorganisms in the world around us are
so large that infection is a daily event.
6. Sickness is not a daily event because not all infections
7. Infectious disease results only when the growth of a
pathogen begins to injure the cells and tissues of an
8. The relationship between a pathogen and the organism it
infects is essentially that of a parasite and its host.
9. A parasite is an organism that obtains nutrition from the
body of the host in a way that harms the host.
10. The parasitic lifestyle of the pathogen enables it to take
advantage of the host and to ultimately become dependent
upon the host organism for its survival.
SPREAD OF DISEASES
1. Many pathogens are present in the environment and
require only the opportunity to enter the body to produce
2. Some infectious disease-such as the common cold,
measles, mumps, and influenza (flu) are spread from one
person to another through coughing and sneezing
3. Other infectious disease spread through contaminated
water supplies or food that has been handled by people
infected with a disease.
4. Infected animals such as ticks and mosquitoes spread other
5. Sexual contact is another way in which diseases are
THE GERM THEORY OF INFECTIOUS DISEASE
1. For thousands of years people believed that evil spirits,
magic, or vapors rising from marshes or decaying plant or
animal matter caused diseases.
2. People actually feared that those who became ill were
cursed or had brought bad luck with them.
3. A new idea developed in the nineteenth century explained
the origins of infectious diseases, based on the work of
French chemist Louis Pasteur and the German physician
Robert Koch; it was shown that microorganisms caused
4. This idea is now known as the germ theory of infectious
5. Koch’s experiments and observation led him to develop a
series of rules for proving that a specific type of
microorganism causes a specific disease. These steps are
called Koch’s postulates:
a. The suspected pathogen must occur in the body of an
animal with the disease and not in the body of a
b. The suspected pathogen should be isolated and grown
in a laboratory culture. (Pure culture)
c. When the microorganisms grown in pure culture are
injected into a healthy animal, the animal should
develop the disease.
d. The pathogen from the second animal should be
isolated and grown in the laboratory. It should be the
same as the pathogen isolated from the first animal.
(Throat culture - strep throat- caused by a bacterium
6. Koch’s postulates are still in use today in the study of
SKIN AND MUCOUS MEMBRANES - "the first line of
1. Although infectious diseases are caused by many different
pathogens, most infectious diseases are spread in one of
A. Through direct contact with an infected person
B. Through indirect contact with an infected person
(coughing or sneezing).
C. Through contaminated food or water.
D. Through the bite of and infected animal.
Disease Pathogen Method of Transition
Athlete’s foot Fungus Contact with contaminated objects
Common cold Virus Close contact with infected person;
Hepatitis A Virus Person to person contact; eating or
drinking contaminated food
Influenza Virus Person to person contact; contact
with contaminated objects
Malaria Protozoa Mosquitoes; transfusion with
Measles Virus Contact with infected mucus or
Pneumonia Virus or Contact with infected mucus or
Syphilis Bacteria Sexual contact
2. Infectious diseases that can be spread from one person to
another are called contagious diseases.
3. The immune system is our primary defense against disease
4. Nonspecific defenses are the body’s first line against
disease. They are not directed against a particular
5. Nonspecific defenses guard against all infections,
regardless of their cause.
6. Specific defenses are attempts by the body to defend it
against particular pathogens.
7. The body’s most important nonspecific defense is the skin.
Unbroken skin provides a continuous layer that protects
almost the whole body. Very few pathogens can penetrate
the layers of dead cells at the skin’s surface.
8. Oil and sweat glands at the surface of the skin produce a
salty an acidic environment that kills many bacteria and
9. Infections are a result of the penetration of the broken skin
by microorganisms normally present on the unbroken skin.
10. Pathogens also enter the body through the mouth and nose,
but the body has nonspecific defenses that protect those
11. Mucous membranes are epithelial tissues that protect the
interior surfaces of the body that may be exposed to
12. Mucous membranes serve as a barrier and secret mucus, a
sticky fluid that traps pathogens.
13. Mucus, cilia, and hairs in the nose and throat trap viruses
and bacteria. Cilia in the trachea trap bacteria; stomach
acid and digestive enzymes destroy pathogens that make it
to the stomach.
14. Many secretions of the body, including mucus, saliva,
sweat, and tears, contain lysozyme, an enzyme that breaks
down the cell wall of many bacteria.
THE INFLAMMATORY RESPONSE "the second line of
1. Despite the initial defenses of the skin and mucous
membranes, pathogens sometimes enter the body.
2. The body’s second line of defense acts when tissues are
3. The injured cells release a chemical called histamine,
which starts a series of changes called the inflammatory
response. This second line of defense is called the
4. Histamine increases blood flow to the injured area and
increases the permeability of the surrounding capillaries,
as a result, fluid and white blood cells (WBC) to leak from
blood vessels into nearby tissue.
5. Pathogens are attacked by phagocytes, which are WBCs
that engulf and destroy pathogens by phagocytosis.
6. The most common type of phagocyte is the, 50 to 70
percent of the white blood cells in the body, is the
7. Neutrophils circulate freely through blood vessels, and
they can squeeze between cells in the walls of a capillary
to reach the site of infection. They then engulf and destroy
any pathogens they encounter.
8. Another type of phagocyte is the macrophage (“big
eater”), they consume and destroy any pathogens they
encounter, they also rid the body of worn out cells and
9. Some macrophages are stationed in the tissues of the body,
awaiting pathogens, while others move through the tissues
and seek out pathogens.
10. Natural killer cells are large white blood cells that, unlike
phagocytes, attack cells that have been infected by
pathogens, not the pathogen themselves.
11. Natural killer cells are particularly effective in killing
cancer cells and cells infected with viruses.
12. A natural killer cell punctures the cell membrane of its
target cell, allowing water to rush into the cell, causing the
cell to burst. (cytolysis)
13. If the infection remains small and in one place, a reddish
swollen area develops just beneath the skin. The area is
said to be inflamed ("on fire"). The familiar symptoms of
inflammation caused by the release of histamine.
14. A serious infection may allow pathogen to spread
throughout the body. The immune system now responds
in two ways:
a. It produces more WBCs.
b. It releases chemicals that stimulate the actions of these
white cells by increasing temperature. Causes a
fever. (Elevated body temperature above normal 37
°C or 98.6 °F)
15. Physicians know that a fever and an increase in WBCs are
two indications that the body is fighting infection.
16. Fever is not a disease; it is a sign that the body is
responding to an infection.
17. Fever also serves another important function: many
diseases causing microorganisms can survive within only a
narrow temperature range. A fever can often slow down or
stop the growth of some microorganisms.
18. In general, body temperatures greater than 39 °C or 103 °F
are considered dangerous, and those greater than 41°C or
105 °F are often fatal.
INTERFERON - defense against viruses
1. Two components of the immune system fight only
viruses: interferon and natural killer cells.
2. Interferon is a protein that interferes with the replication
of viruses. Cells that have been invaded by viruses release
3. Although interferon cannot save an invaded cell, it works
as a warning signal for healthy cells, in which it interferes
with viral replication. These effects on a virus slow down
the progress of infection and often give the specific
defenses of the immune system time to respond.
4. Natural killer cells, which also defend the body against
viruses, are contained in blood and lymph. These cells
attack body cells that have been infected by viruses.
Because viruses can only replicate in a host cell, killing
the host cell also destroys the virus.
5. Interferon and natural killer cells also help to fight against
If a pathogen is able to get pass the body’s nonspecific
defenses, the immune system reacts with a series of specific
defenses that attack the disease-causing agent. The specific
defenses of the body are collectively known as the immune
There are two types of immunity:
1. Active immunity - the immunity produced by a vaccine is
known an active immunity because the body has the ability
to mount an active immune response against the pathogen.
The injection of a weakened or mild form of a pathogen to
produce immunity is known as vaccination. (Long-lasting)
2. Passive immunity - if antibodies produced by other
animals against a pathogen are injected into the
bloodstream, they produce passive immunity against the
pathogen as long as they remain in the circulation, usually
for several weeks. (Short-term)
1. An allergy is the response of the immune system to a
normally harmless substance as if it were an antigen.
2. Allergies result when antigens bind to mast cells, which
are a type of immune cell found throughout the body but
especially in the linings of the nasal passage.
3. When allergy-causing antigen attaches to mast cells, the
activated mast cells release chemicals known as
4. Histamines increase the flow of blood and fluids to the
surrounding area, and produce sneezing, runny eyes and
nose, and other irritations that makes a person with
5. Antigen on plant pollen, dust, molds, and animal fur
6. One of the most serious allergic reactions is asthma, a
condition in which smooth muscles contract around the
passage to the lungs, making breathing difficult.
7. Asthma and allergies can usually be treated with
antihistamines (reverse the effects of histamines) drugs
and other medicines.
1. When the immune system is functioning normally it
distinguishes "self" from "non-self".
2. But sometimes the immune system mistakes its own cells
for pathogens, resulting in an autoimmune disease.
3. In an autoimmune disease, the immune system attacks the
tissue of the body.
4. Several diseases are caused by immune disorders:
a. Rheumatic fever resulting from Strep Throat causing
cells death and scarring to the heart lining and the
b. Rheumatoid arthritis a destructive inflammation of
c. Juvenile diabetes reaction against the insulin
producing cells of the pancreas.
d. Multiple sclerosis a nerve disease that results from
the destruction of myelin sheath of nerve fibers.
AIDS - acquired immunodeficiency syndrome
The immune system normally provides very effective
protection against infectious diseases. The diseases in which
the immune system malfunctions dramatically illustrate its
importance to our health. The most deadly of these diseases is
AIDS, or acquired immunodeficiency syndrome. Aids was
first recognized as a disease in 1981, and since then it has
killed more than 300,000 Americans.
1. AIDS stands for acquired immunodeficiency syndrome.
AIDS is the most serious stage of a condition that disrupts
the normal function of the immune system.
2. AIDS is not a specific disease, but a condition in which the
immune system cannot protect the body against a variety
3. The human immunodeficiency virus or HIV causes AIDS.
HIV attacks the human immune system and destroys the
body’s ability to fight infection.
4. Once HIV enters the body, it attaches to receptors on the
surface of a type of T Cell known as helper T Cells (T4
Cells). These cells are so named because they help other
lymphocytes respond to the early stages of an infection.
5. As HIV destroys helper T Cells, the body loses its ability
to fight off disease. HIV destroys the body’s ability to
THE TRANSMISSION OF HIV
1. HIV can only be spread from one person to another only
through direct contact of certain body fluids, such as
blood, semen, or breast milk.
2. HIV is not spread by casual contact, such as holding hands
or using dishes previously used by a person with HIV.
3. It is also not transmitted by insects (mosquitoes, fleas, or
4. Most often, HIV is transmitted through sexual intercourse,
sharing needles, or by transfusion of contaminated blood.
5. HIV is now the fastest growing epidemic in the world.
6. According to the United Nations aids project, more than 22
million people are infected with HIV.
7. Educating yourself about HIV can help reduce your risk of
getting the disease.
8. Condoms reduce the risk of transmitting HIV from one
person to another during sexual intercourse.
9. Sharing needles with others is one of the main ways the
virus is transmitted from person to person.
10. You never risk getting HIV while donating blood. Blood
is routinely tested for HIV, and this has almost eliminated
the risk of getting HIV from blood transfusions.
11. There is no known cure for an HIV infection.
12. At present, the best way to avoid an HIV infection is to
learn as much as you can about the ways HIV is
13. Avoid behaviors that place you at risk of contacting the
STAGES OF HIV INFECTION
1. An HIV infection progresses on a fairly predictable
course. Each stage of the infection is associated with
certain symptoms, but the timing of the stages varies with
2. When first infected by the virus, a person may have flu
like symptoms or no symptoms.
3. Within a few weeks to several months, antibodies to HIV
begin to appear in the blood.
4. The presence of antibodies in the blood is used to diagnose
the disease and to screen donated blood.
5. A person is said to be HIV positive when HIV antibodies
are present in the blood. Even when a person is diagnosed
as being HIV positive, other symptoms of the syndrome
may not appear for months or even years.
6. In time, HIV attacks and destroys helper T cells, the helper
cells that activate the immune system to fight infection.
7. At first, an HIV positive person may experience a phase of
mild symptoms including fever, weight loss, and swollen
8. When the number of helper T cells in the blood become so
low (below 200/ml) that the immune system cannot fight
disease, an HIV infection has developed in aids.
9. The length of time it takes for an HIV infection to becomes
aids varies from person to person, but it may take from a
few to 15 to 20 years.
10. People with aids may have a variety of diseases. Typically
people with aids become sick when they are infected by
pathogens that do not normally cause disease in people
with healthy immune systems. These diseases are called
11. An HIV infection is generally considered fatal. However,
not everyone who has been diagnosed as HIV positive has
12. People with aids die when their body’s weakened immune
system cannot fight off an opportunistic infection caused
by a pathogen.
13. Few individuals live more than two years after being
diagnosed with aids, usually dying from opportunistic
infections or cancer.
14. Scientists trying to create vaccines for HIV must contend
with its very rapid rate of evolution. The genes that code
for the virus’s surface proteins mutate frequently.
15. HIV’s rapid evolution also complicates the task of
developing treatments (drugs) for HIV infection and aids.
16. The virus quickly becomes resistant to drugs used against
it. To avoid this problem, scientists have begun treating
patients with three antiviral drugs at once.