8.1 Catching it
4. Explain how bacteria and viruses can disrupt regular cellular or body function.
5. Explain how immune systems respond to infections.
Readings: 18:476 -487
Reminders: Assignment # 3 Salmon Farming is due on Wednesday Oct 28 You must have an ID# from Turnitin written
on the paper copy you submit
How do bacteria and viruses infect an organism
-They get past these defenses through injury, insect bite, sheer numbers, virulence (severity),
weakened immune system
What protects us from pathogens?
1.Innate (nonspecific) defenses
First line of defense:
-skin (layer of dead cells) acts as a barrier
-mucus membranes: nose, mouth, ears, esophagus, anus, vagina, penis.
Secreted mucus traps many foreign cells and substances, fine hairs called cilia
move trapped substances to exit.
-lysozyme (an enzyme found in tears, sweat, saliva which digests many bacterial cell walls)
-stomach acid (very low pH degrades many molecules, including some cell membranes)
-competitive exclusion from our normal bacteria
QUESTION: 1.What in the first line of defense can prevent you from getting these diseases? Match each of
these to the 5 answers below.
1. cold virus – (b) Mucus membranes
2. food poisoning - (d) Stomach Acid
3. Candida sp. yeast infection of the urogenital system – (e) Competitive exclusion by normal bacteria
5. Athlete’s foot – (a) Barrier layer of dead skin cells
a. Barrier layer of dead skin cells
b. Mucus membranes
d. Stomach acid
e. Competitive exclusion by normal bacteria
Second line of defense:
Phagocytes (eg. macrophages) engulf microbes or virus-infected cells
with pseudopodia. They extend their cytoplasm quickly into extensions of
their plasma membrane. This in effect a form of endocytosis. Once
engulfed enzymes inside the macrophage help destroy the pathogen.
Macrophages circulate in our lymph system and when then pathogens
need to be destroyed it tends to happen in lymph nodes.
Natural killer cells
These are a type of white blood cell (not a B or T cell) that circulate through the body and recognize when a
cell has been infected by a virus. Such cells release chemical signals (interferons and cytokines). Natural killer
cells then secrete several molecules themselves which poke holes in the plasma membrane of the infected
cells and then cause the cell to initiate its own destruction (programmed cell death).
QUESTION: 2. Macrophages and natural killer cells are both types of white blood cells. They are both attack
pathogens in a non-specific type way. What is different about the way they destroy pathogens?
These proteins circulate in the blood and recognize foreign microbes. They cover these bacterial or
eukaryotic invaders and can poke holes in the cells causing them to burst. By covering pathogen
cells they can also assist macrophages to engulf these cells. Complement proteins can also assist
some specific-targeted defenses.
QUESTION: 3. Although natural killer cells don’t produce complement proteins what is similar about how they
Damaged cells release
This causes nearby blood
vessels to become leaky
releasing white blood cells
into fluid between cells of
affected area. An increase in
blood flow causes
redness, swelling, heat.
It occurs with systemic infections eg. toxins are released into the bloodstream by pathogens or the pathogens
themselves are in the bloodstream
Fever can be caused by the toxins interacting with macrophages or by macrophages alone. In either case a
chemical signal is sent to the brain to stimulate the body to increase body temperature. Macrophages and
some other white blood cells can multiply and work more quickly at higher temperatures.
Question: 4. Remembering what you know about ecological niches how could higher temperatures caused
by fevers adversely affect some pathogens?
2. Specific immunity (specific pathogens are recognized as foreign by lymphocytes and
Third line of defense: Immune and lymphatic system
Lymph moves invaders from the
sites of infection into lymph nodes
where they are attacked by masses
of white blood cells. This is why
your lymph nodes ie in your neck
can swell up when you are sick
Besides macrophages and natural
killer cells which are non-specific,
lymph contains some lymphocytes
called B cells and T cells which
provide specific protection.
Note: on the importance of cell surface proteins
Proteins are found on the surface of virtually all cells and
serve very many functions.
Receptor proteins are used by cells to sense and
respond to incoming signals ie “Grow in this direction”
“Ouch!” “Take in more potassium ions” “Work harder”
“Turn on gene X” “Beware of invaders” “Die!”
A ligand is what binds to a receptor to signal it. Two
receptor proteins(R) with bound ligands (L) are shown on
the left. Only 1 ligand binds to1 receptor as a rule. They
act like lock-and-key. There are many different types
of cells in your body with different
receptors. As well, cells in different species have different cell surface receptor proteins. There are 1000s of
different locks and keys.
The way immune system cells and antibodies work is by recognizing foreign substances called antigens. The
cells of your immune system recognize the proteins on the surface of your own cells as being “self” and leave
them alone. Any shape on an antigen that is recognized as foreign (non-self) is attacked by the immune
system. Antigens could be dust, bacteria, viruses, or toxins.
QUESTIONS: 5. How do prions evade the immune system?
6. What has gone wrong with the immune system when a person has an auto-immune disease?
Three major players in your immune system are:
B cells T cells antibodies
Antibodies are specialized proteins secreted by B cells.
Antibodies also recognize epitopes and assist the immune
system in many ways.
Both B and T cells are types of lymphocytes or
white blood cells. Both have receptors on their
surface to recognize epitopes.
One B cell only recognizes and binds to one epitope (specific
shape) Also, one antibody or one T cell only recognizes one
epitope. At any one time your body is making thousands of B
cells, T cells and antibodies to be able to recognize thousands of
potential foreign invaders. The antigen receptors on the surface of
T cells are basically the same thing as antibodies.
Once an invader is detected, the cells of the immune
system that recognize the invader multiply to combat
If your body has successfully recognized an antigen and
made antibodies and white cells against it, you usually
don’t get sick from that specific antigen a second time
because your body responds much more quickly the