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IMMUNE SYSTEM
6.3 Defense against infectious
disease
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Define pathogen.
Explain why antibiotics are effective against bacteria but not against viruses.
Outline the role of skin and mucous membranes in defense against pathogens.
Outline how phagocytic leucocytes ingest pathogens in the blood and in body
tissues.
Distinguish between antigens and antibodies.
Explain antibody production
Outline the effects of HIV on the immune system.
Discuss the cause, transmission and social implications of AIDS
Pathogens and Infectious
Infectious disease (communicable disease) is
caused when another organism or virus invades
the body (host) and lives there parasitically.
Pathogen: is an organism or a virus that causes
disease. Usually they are microorganisms.
Vector: an organism that transmits a disease
causing organism, or a device for transferring
genes during genetic engineering.
What are the non-communicable diseases?
PATHOGENS
1- Bacteria
2- Fungi
3-Viruses
4-Protozoa
5-Some invertebrate animals (flatworms,
roundworm ……….)
IMMUNE SYSTEM ORGANS
Spleen: The organ of the immune
system that is located in the
abdomen cavity.
• Degradation of old RBCs
• Storage of blood as a reserve in
the event of any shortage
• Production of lymphocytes
• Production of fetal blood until
birth
Can we survive without our
spleen????
Lymph nodes: Lymph nodes
filter the lymphatic fluid
and store special cells that
can trap cancer cells or
bacteria that are traveling
through the body in the
lymph fluid
Tonsils:
The tonsils trap bacteria and
viruses entering through the
throat and produce
antibodies to help fight
infections.
Thymus Gland:
• The site of T lymphocyte
cell differentiation.
• The thymus increases
gradually in size and activity
until puberty, becoming
vestigial (useless)
thereafter.
What is immunity?
It is the recognition and removal of molecules, cells
that foreign to the body. Immunity can be gained by
two ways.
There are two types of immunity;
1) Non-specific immunity (innate immunity)
2) Specific immunity (acquired immunity)
1- Non-specific immunity
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The basic resistance to disease that a species
possesses - the first line of defense against
infection.
Skin barrier
HCl in the stomach
Lysozyme enzyme in sweat/saliva ( to digest
bacteria)
Mucous membranes protecting mouth, anus,
genitals.
Phagocytic leucocytes.
SKIN
• It is the first line of defense. External skin is
covered by dry dead cells with keratinized
proteins.
• Folds or moist parts of the skin are good
habitat for pathogens.
Mucus
• Inner lining of the body (respiratory tract and
digestive tract) are covered by mucosa layer
that secrete mucus for protection.
Phagocytic WBS
• Granulocytes, Monocytes
• Interferons  protein that is produced by
body cells to fight with viruses.
Sythesized proteins inhibit the synthesis of
some enzymes that are required for virus
replication.
• Inflammation The local response to injury,
involving small blood vessels, the cells
circulating within these vessels, and nearby
connective tissue. Fever occurs.
White blood cells are fighting with
bacteria.
2) Specific Immunity
(acquired-adaptive immunity)
It is provided by
lypmhocytes which are
specific to the bacteria.
a) B lymphocytes: synthesize
antibodies. Antibodies destroy
bacteria (antigen). ….
b) T lymphocytes: they kill the
infected body cells to prevent
spread the infectious.
How do the cells work?
(Active or Passive immunity)
Active immunity
• Gained by either
direct contact with
pathogen or get
vaccine shot.
• Our body produce
special antibodies
against that
pathogen.
Passive immunity
• Gained by taking
antibodies from another
organisms (mother, serum
from other animals).
• Our body does not
produce antibody, instead
it uses antibodies that
come from other
organisms. It is short term
immunity.
• Ex. Serum which is used in
snakebite
ANTIBODIES ARE BIOLOGICAL WEAPONS.
HOW DO THEY KILL THE PATHOGEN?
ANTIBODIES
• Y-shaped proteins
• Produced by B cells
• Antibody recognizes the pathogen/antigen and
binds to it (tagging it)
• This causes one of two things to happen:
1) Macrophages (killer cells) to come and ‘eat’ the
bacteria (phagocytosis)
Or 2) Other antibodies come and help out and kill the
bacteria directly (cell lysis)
ANTIBIOTICS or ANTIBACTERIAL
• Antibiotics are used to treat infections caused by
bacteria. (ex. Penicillin)
• A substance that kills or slows down the growth of
bacteria.
• There is concern worldwide that antibiotics are
being overused. Antibiotic overuse is one of the
factors that contributes towards antibiotic
resistance as the growing number of bacterial
infections which are becoming resistant to
antibacterial medications.
How do antibiotics kill bacteria?
Remember bacterial life cycle!
Antibiotics stop bacterial reproduction by blocking:
1- DNA polymerase
2- RNA polymerase
2- Enzyme for cell wall synthesis
Are antibiotics effective against viruses?
Think about viral life cycle!!
VIRUSES
• Viruses are known as ‘genes in a box’.
• They are made of protein coat (capsid) and
and a nucleic acid either DNA or RNA but not
both of them.
• They do not have metabolic reactions and can
not reproduce themselves.
• They need a host cell to reproduce.
Viruses have two types of reproductive cycles.
1. In the lytic cycle,
– viral particles are produced using host cell components,
– the host cell lyses, and
– viruses are released.
2. In the Lysogenic cycle
– Viral DNA is inserted into the host
– Viral DNA is duplicated along with the host chromosome during
each host cell division.
– The inserted phage DNA is called a prophage.
– Most prophage genes are inactive.
– Environmental signals can cause a switch to the lytic cycle,
causing the viral DNA to be excised from the bacterial
chromosome and leading to the death of the host cell.
© 2012 Pearson Education, Inc.
Figure 10.17_s2
Phage
Attaches
to cell
Phage DNA
4
The cell lyses,
releasing
phages
1
Bacterial
chromosome
The phage injects its DNA
7
Lytic cycle
Phages assemble
Environmental
stress
Lysogenic cycle
2
The phage DNA
circularizes
Prophage
6
The lysogenic bacterium
replicates normally
OR
3
Many cell
divisions
New phage DNA and
proteins are synthesized
5
Phage DNA inserts into the bacterial
chromosome by recombination
Figure 10.17_1
Phage
Attaches
to cell
Phage DNA
4
The cell lyses,
releasing
phages
1
Bacterial
chromosome
The phage injects its DNA
Lytic cycle
Phages assemble
2
3
The phage DNA
circularizes
New phage DNA and
proteins are synthesized
Figure 10.17_2
Phage
Attaches
to cell
Bacterial
chromosome
Phage DNA
1
The phage injects its DNA
7
Environmental
stress
Many cell
divisions
Lysogenic cycle
2
The phage DNA
circularizes
5
Prophage
6
The lysogenic bacterium
replicates normally, copying the
prophage at each cell division
Phage DNA inserts into the bacterial
chromosome by recombination
Figure 10.20A
HIV VIRUS (human immunodeficiency virus)
Envelope
Glycoprotein
Protein coat
RNA
(two identical
strands)
Reverse
transcriptase
(two copies)
The AIDS virus makes DNA on an RNA template
 AIDS (acquired immunodeficiency syndrome) is caused
by HIV (human immunodeficiency virus).
 HIV
• is an RNA virus,
• has two copies of its RNA genome,
• carries molecules of reverse transcriptase, which causes
reverse transcription, producing DNA from an RNA template.
© 2012 Pearson Education, Inc.
The AIDS virus makes DNA on an RNA template
 After HIV RNA is uncoated in the cytoplasm of the host
cell,
1. reverse transcriptase makes one DNA strand from RNA,
2. reverse transcriptase adds a complementary DNA strand,
3. double-stranded viral DNA enters the nucleus and integrates
into the chromosome, becoming a provirus,
4. the provirus DNA is used to produce mRNA,
5. the viral mRNA is translated to produce viral proteins, and
6. new viral particles are assembled, leave the host cell, and can
then infect other cells.
© 2012 Pearson Education, Inc.
Figure 10.20B
Reverse
transcriptase
Viral RNA
1
DNA
strand
CYTOPLASM
NUCLEUS
Chromosomal
DNA
2
Doublestranded
DNA
3
Provirus
DNA
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5
Viral
RNA
and
proteins
RNA
6
Effect of HIV on the immune
system: AIDS
• HIV host cells are antibody secreting
lymphocytes.
• HIV takes control of lymphocytes, and their
number decreases.
• So, patient becomes vulnerable to attack by
opportunistic infectious (pneumonia,
meningitis, cancers that would normally be
resisted by a person with a healthy immune
system.
Treatment and prevention of AIDS
• Until there is a vaccine or a cure, the best way
to stop AIDS is to educate people about how
the virus is transmitted.
• HIV mutates very quickly.
• New strains are resistant to AIDS drugs.
• Drug-resistant strains now infect new patients.
The Social Consequence of AIDS
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Psychological
Economic
Child development and education
Child health.
Medical services provision
National factors