AP BIOLOGY ANIMAL
FORM AND FUNCTION
Immune System
Immune System
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Animals are constantly under attack by pathogens,
infectious agents that cause disease.
In response, animals fight back in various ways.
Natural Barriers Prevent Infection
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1. Skin (or shells)
2. Chemical secretions that trap or kill microbes.
Example: Saliva contains lysozyme, an enzyme that
digests microbial cell walls. Also found in tears.
3. Mucus membranes line the passageways of body.
Mucus traps microbes and other particles—sent to
digestive system.
Non-specific Defenses
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These immune defenses protect against any type of
invader that may get past the barriers provided by
skin and mucus membranes.
These are not specific to a particular pathogen.
Includes: Phagocytes, Complement Proteins, and the
Inflammatory Response.
Phagocytes
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If an invader does get into the body, it may
meet special cells called phagocytes. “phago”
= eating. These cells roam the body in search
of bacteria and dead or damaged cells. 3
main types: Granulocytes (Neutrophils),
Macrophages, and Dendritic cells.
Phagocytes—Granulocytes
(Neutrophils)
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Granulocytes: (also called Neutrophils) often take the
first stand during an infection. They attack any
invaders in large numbers, and "eat" until they die.
The pus in an infected wound consists chiefly of dead
granulocytes. A small part of the granulocyte
community (called Eosinophils) is specialized in
attacking larger parasites such as worms.
Phagocytes--Macrophages
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The macrophages ("big eaters") are slower to
respond to invaders than the granulocytes, but they
are larger, live longer, and have far greater
capacities. Macrophages also play a key part in
alerting the rest of the immune system of invaders.
Macrophages start out as white blood cells called
monocytes. Monocytes that leave the blood stream
turn into macrophages.
Phagocytes—Dendritic Cells
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The dendritic cells are "eater" cells and devour
intruders, like the granulocytes and the
macrophages. And like the macrophages, the
dendritic cells help with the activation of the rest of
the immune system. They are also capable of
filtering body fluids to clear them of foreign
organisms and particles
Complement Proteins
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Phagocytes are aided by special protein molecules
called complement proteins.
Some complement proteins poke holes in bacterial
cell walls.
Complement proteins may
coat bacterial cells,
stimulating phagocytes
to ingest them.
Complement Proteins Aid Immune
Response
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These proteins flow freely in the blood and can
quickly reach the site of an invasion where they can
react directly with antigens - molecules that the
body recognizes as foreign substances. When
activated, the complement proteins can:
Trigger inflammation
Stimulate phagocytes
Coat pathogens
Kill pathogens
Inflammatory Response
Let’s say you pick up a splinter as you grab a piece of wood.
Ouch! Your immune system will respond with an inflammatory
response.
 Within our tissues lie cells known as Mast Cells.
 These cells contain the signal histamine, a chemical that
activates the inflammatory response.
 The splinter damages these mast cells, causing them to release
histamine, which then
migrates through the tissues towards
the bloodstream.
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Inflammatory Response
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The release of histamine causes increased
permeability and blood flow to the injured tissue.
The splinter also causes the release of signals to
phagocytes which go to the site and “eat”
bacteria
and
cell debris.
Lymphatic System
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The lymphatic system consists of lymphatic vessels, through
which lymph travels, and various structures that trap “foreign”
molecules and particles.
Lymphatic Cells—Important for
Immune Response
The lymphatic system also involves a transportation
system - lymph vessels - for transportation and
storage of lymphocyte cells within the body. The
lymphatic system feeds lymphatic cells into the body
and filters out dead cells and invading organisms
such as bacteria.
 Two main kinds
of lymphocytes:
B cells and T cells
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Acquired Immune Response—B and
T Cells
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B cells and T cells, types of white blood cells called
lymphocytes, are critical for the acquired immune
response.
Lymphocytes that originate from stem cells in the
bone marrow, then migrate to the thymus, then
mature into T cells (“T” for thymus).
Lymphocytes that originate and mature in the bone
marrow are B cells (“B” for bone marrow).
Antigens
An antigen is a molecule that is foreign to the body
and causes the immune system to respond.
 B and T cells recognize antigens using the antigenspecific
receptors
embedded
in their plasma
membranes.
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Antigen receptors
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Every B and T cell has a specific antigen recognition
site on its surface. Each person has more than 1
million different B cells and 10 million different T
cells, each with a particular antigen-binding
specificity.
Helper T cells
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Helper T cells are the major driving force and the
main regulators of the immune defense. Their
primary task is to activate B cells and killer T cells.
Activating Helper T Cells
Helper T cells themselves must be activated. This
happens when a macrophage or dendritic cell,
which has eaten an invader, travels to the nearest
lymph node to present information about the
captured pathogen.
 The phagocyte displays an
antigen fragment from the
invader on its own surface,
a process called antigen
presentation.
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Activating the Helper T cell
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When the receptor of a helper T cell recognizes the
antigen, the T cell is activated. Once activated,
helper T cells start to divide and to produce
proteins that activate B and T cells as well as other
immune cells.
Antigen Presentation to Activate the
Helper T Cells
B cell activation
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The B lymphocyte cell searches for antigen matching
its receptors. If it finds such an antigen it connects to
it, and inside the B cell a triggering signal is set off.
The B cell now needs proteins (called cytokines)
produced by helper T cells to become fully activated.
When this happens, the B cell starts to divide to
produce clones of itself.
During this process, two new cell types are created,
plasma cells and B memory cells.
B Cell activation
Plasma Cells Produce Antibodies
The plasma cell is specialized in producing a
specific protein, called an antibody, that will
respond to the same antigen that matched the B cell
receptor.
 Antibodies are released from
the plasma cell so that they
can seek out intruders and
help destroy them.
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Production of Antibodies
Plasma cells
produce
antibodies at an
amazing rate
and can release
tens of thousands
of antibodies
per second.
Antibodies
When the Y-shaped antibody finds a matching
antigen, it attaches to it.
 The attached antibodies serve as an appetizing
coating for eater cells such as the macrophage.
 Antibodies also neutralize
toxins and incapacitate
viruses, preventing them
from infecting new cells
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How do antibodies recognize the
antigen?
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Antibodies are protein molecules with two functional
regions:
1. fragment antigen binding region or Fab---this is
what allows an antibody to recognize a specific
antigen.
2. The other end, which binds to effector cells
(immune cells), is called the Fe region.
There are 5 types of Fe regions, one for each of the
five types of antibodies: IgA, IgD, IgE, IgM, and
IgG.
Structure of Antibodies
1. Fab binds to antigen
2. Fe binds to immune cell
that destroys antigen
3. heavy chain
4. light chain
5. antigen binding site
6. hinge regions
Memory B and T Cells
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The Memory Cells are the second type of cell
produced by the division of B cells. These cells have a
prolonged life span and can thereby "remember"
specific intruders.
T cells can also produce
memory cells with an
even longer life span
than B memory cells.
Memory B and T cells
The second time an intruder tries to invade the
body, B and T memory cells help the immune
system to activate much faster.
 The invaders are wiped out before the
infected human feels any symptoms.
 The body has achieved
immunity against the
invader.
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This graph shows how Memory Cells help you to better fight
infections. At day 0, someone catches a virus. At day 10, her B-cells
start making antibodies, and by day 15 she’s made enough
antibodies to destroy all the viruses. Now, she doesn’t make any
more antibodies, so fewer and fewer are left in her body. Then, at
day 40, the same virus gets in her body again. Since she has
Memory B-cells prepared to fight, she can quickly make 100 times
more antibodies than she did during the first infection.
Acquired Immune Response &
Immunological Memory
Not only do B cells and T cells recognize and
inactivate foreign cells and molecules.
 Both also contribute to immunological memory, an
enhanced response to a pathogen encountered
previously.
 Vaccines are given to produce
an immune response to an
inactivated microbe. This
provides immunological
memory.
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Killer T cells (Cytotoxic cells)
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The killer T cell (cytotoxic cell) is specialized in
attacking cells of the body infected by viruses and
sometimes also by bacteria. It can also attack
cancer cells. The killer T cell has receptors that are
used to search each cell that it meets. If a cell
is infected, it shows
a tiny piece of the
antigen and it is
swiftly killed.
Natural Killer Cells
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Natural Killer (NK) cells help recognize and
eliminate certain diseased cells.
All cells in the body (except RBCs) normally have on
their surface a protein called a class 1 MHC
molecule. Following viral infection or conversion to a
cancerous state, cells sometimes stop expressing this
protein.
The NK cells that patrol the body attach to such
stricken cells and release chemicals that lead to cell
death, inhibiting further spread of the virus or
cancer.
When Things Go Wrong….
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Allergies—Occur when the immune system
overreacts to the presences of certain antigens. This
causes an overwhelming release of histamines and
a (sometimes extreme) inflammatory response.
HIV/AIDS
Human Immunodeficiency Virus (HIV) attacks the
Helper T cells of the immune system. Since the Helper
T cells regulate the immune system response, without
them the person is susceptible to any pathogen.
 When Helper T cell numbers
reach a low enough level,
the person is diagnosed with
AIDS (Acquired Immune Deficiency Syndrome). This
virus is spread with direct contact in the blood stream
with infected body fluids.
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