Defense Against Infectious
Disease
How does my body fight disease?
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Pathogen
• An organism or virus
that causes disease.
• This includes virus,
bacteria, protozoa,
fungi and worms of
various types.
Tripanosoma cruzi protozoans in blood – Chagas disease
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• Not every pathogen that
enters our body results
in disease
• This is because for most
pathogens, our body has
developed immunity.
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• There are also
chemicals called
antibiotics that work
against bacterial cells
without affecting our
body cells.
How antibiotics work
• Saprotrophs, including bacteria and fungi, compete
with each other for death organic matter on which
they feed.
• To inhibit the growth of their competitors, some
saprotrophs produce antibiotics.
• An antibiotic is a chemical that inhibits the growth
of microorganisms.
• Antibiotics produced by fungi mostly inhibit
bacteria.
• There are smaller number of antibiotics produced by
bacteria that inhibit fungi.
How antibiotics work
• Antibiotics
interfere with
various
metabolic
processes
• Antibiotics are
not effective
against viruses.
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Organism that produces Antibiotic produced
the antibiotic
Fungi:
Mechanism of action
Penicillium sp.
Penicillin
Cehalosporium sp.
Cephalosporin
Streptomyces sp.
Tetracycline
Streptomycin
Inhibits cell wall
synthesis
Inhibits cell wall
synthesis
Inhibits translation
Saccharopolyspora
erytraea
Bacteria:
Erytromycin
Inhibits translation
Bacillus sp.
Polymyxing
Bacitracin
Disrupts cell membranes
Disrupts cell wall
synthesis
Preventing pathogens from entering
our bodies.• The ability to resist infection by a disease is
termed immunity.
• There are two types of immunity:
▫ Non specific immunity allows the body to resist
infection by a wide range of pathogens.
▫ Specific immunity allows the body to resist a
specific pathogen
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Non-specific immunity
First line of defense:
Tough: provides a physical barrier against the entry of pathogens and
protection against physical and chemical damage

Skin

Sebaceous glands Secrete a chemical called sebum, which maintains skin moisture and
and hair follicles slightly lowers skin pH to inhibit the growth of bacteria and fungi.

Mucous
membranes
Line internal cavities that are exposed to the external environments
such as nasal passages.
Secrete mucus, which has antiseptic properties because of the
antibacterial enzyme lysozyme.
Mucus also acts as a physical barrier by trapping pathogens and
harmful particles.
Cilia in some mucus membranes (i.e. trachea) move trapped
pathogens up and out.

Stomach acid
The acidic environment of the stomach helps to kill most of the
ingested pathogens.
Second line of defense
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Non specific immunity
Second line of defense:

Phagocytes
•If a microorganism get past the physical barriers and enter
the body, leucocytes (white blood cells) provide the next line
of defense.
•Macrophages, are large phagocytes that change shape to
surround an invader, taking it in through the process of
phagocytosis and digesting it with enzymes from lysosomes..
•Macrophages can pass through pores in the walls of
capillaries and move to the sites of infection.
•When wounds become infected (septic), large numbers of
phagocytes are attracted, resulting in the formation of pus.
Antibodies are produced in response
to a specific pathogen.• If a microorganisms gets past the physical barriers
and the phagocytes, a second level of immunity
exists.
• Proteins and other molecules on the surface of
pathogens are recognized as foreign by the body and
they stimulate a specific immune response.
Antigen
A foreign substance that stimulates the
production of antibodies
Antibodies
Proteins that recognize and bind to specific
antigens
• The body is
capable of
responding in a
highly targeted
way to
pathogens.
• A protein called
antibody binds
to antigens.
• Our body can
produce a large
variety of
antibodies, each
one specific to a
certain antigen.
• The binding of an antibody to an antigen
produces many benefits, including:
▫ Makes the pathogen more recognizable to
phagocytes
▫ Prevents viruses from docking to host cells so that
they cannot be taken up by host cells.
▫ Cause agglutination (sticking together) of
pathogens so they cannot enter cells and are easier
for phagocytes to ingest.
• Immune response depends on different types of
lymphocytes, each with a specific function. The
process includes:
Macrophage
Ingests the pathogen and displays antigens from it (proteins or
other substances)
T-cells
Identify specific pathogens produced by macrophages and
activate B cells
B-cells
Produce antibodies that specifically attach to the antigen, clone
by mitosis to increase in number, eliminate the pathogen
Memory cells
Some B-cells remain in the bloodstream to give immunity for
a second infection.
http://waynejoseph.files.wordpress.com/2010/03/immune20system.jpg
HIV and AIDS
• The human immunodeficiency
virus (HIV) targets helper Tcells.
• In the early stages of infection,
the immune system makes
antibodies against HIV.
• If these can be detected in a
person’s body, they are said to
be HIV-positive.
http://aids.gov/images/aidsinfographics/what-is-hiv-aids-2.jpg
HIV
• HIV invades and destroys lymphocytes called
helper T-cells resulting in the reduction of their
number and the progressive loss of the capacity
to produce antibodies.
• The rate at which this occurs varies considerably
and it can be slowed down by using antiretroviral drugs.
AIDS
• In most HIV-positive patients antibody production
eventually becomes so ineffective that a group of
opportunistic infections strike.
• Several of these are normally so rare that they are
“marker” diseases for the latter stages of HIV
infection.
• When several infections affect someone due to HIV,
the person is said to have acquired immune
deficiency syndrome (AIDS).
•
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AIDS
• AIDS spreads by HIV infection.
• The virus only survives outside the body for a
short time.
• Infection normally only occurs if there is a blood
to blood contact between infected and
uninfected people.
Ways in which an
infection of HIV
can occur include:
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Social implications:
• Sufferers are stigmatized in terms of
employment, insurance, education
access, social acceptance and others
• families lose members
• Those that die are both parents and
workers
• Grandparents may be left to look after
orphaned children
• Economic productivity is lost
• The costs of treating the disease are
high and may overburden health
services
• In countries where infection rates are
high, there may be an undermining of
social cohesion.
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Sources:
• Allot, A., & Mindorff, D. (2010). IB Biology Course
Companion. New York: Oxford University Press.
• Allot, A., & Mindorff, D. (s.f.). IB Biology Course
Companion 1st ed . Obtenido de Actual Book Pages:
http://aura.edu.in/read/IB/BIOLOGY_COURSE_C
OMPANION_(OXFORD)/original/Page-226.html
• Damon, A., McGonegal, R., Tosto, P., & Ward, W.
(2007). Biology Higher Level. London: Pearson
Baccalaureate.