Transcript The Immune System

Chapter six:
The Immune System
Dr. Sanaa Tork
In this chapter you will learn about :
1. What Is the Immune System and what does It do? The immune
system is the body's defense against infectious organisms and
other invaders.
2. The immune system is made up of a network of cells, tissues,
and organs that work together to protect the body.
3. Distinguish between internal defense mechanisms of
invertebrates and vertebrates
4. Distinguish between specific and nonspecific immune responses
5. The cells that are part of this defense system are white blood
cells, or leukocytes.
6. Leukocytes are produced or stored in many locations throughout
the body, including the thymus, spleen, and bone marrow. For
this reason, they are called the lymphoid organs. There are also
clumps of lymphoid tissue throughout the body, primarily in the
form of lymph nodes, that house the leukocytes. The leukocytes
circulate through the body between the organs and nodes by
means of the lymphatic vessels.
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7. Two basic types of leukocytes combine to seek out and destroy the
organisms or substances that cause disease. The phagocytes: are cells
that chew up invading organisms. The most common type is the
neutrophils which primarily fight bacteria. The lymphocytes: are
cells that allow the body to remember and recognize previous invaders.
8. There are two kinds of lymphocytes: the B lymphocytes and the T
lymphocytes. Lymphocytes start out in the bone marrow and either stay
there and mature into B cells, or they leave for the thymus gland,
where they mature into T cells.
9. B lymphocytes and T lymphocytes have separate jobs to do: B
lymphocytes are like the body's military intelligence system, seeking
out their targets and sending defenses to lock onto them. T cells are
like the soldiers, destroying the invaders that the intelligence system
has identified.
10.Explain the meaning of antigen (a foreign substance that invades the
body and can detect by several types of cells work together to
recognize and respond to it. These cells trigger the B lymphocytes to
produce antibodies)
11.Antibodies are specialized proteins that lock onto specific antigens.
12.Compare active and passive immune system
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13. Disorders of the immune system can be broken down into four
main categories: immunodeficiency disorders (primary or
acquired) , autoimmune disorders (in which the body's own
immune system attacks its own tissue as foreign matter),
allergic disorders (in which the immune system overreacts in
response to an antigen), cancers of the immune system
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Pathogens
-Bactria
-Virus
-Parasite
-Fungi
Foreign invaders - viruses, bacteria, allergens
Immunitybombard our body.
parasites- constantly
Foreign invaders
(Non-self)
Transplantation
Vaccination
Immunity
The ability of organism to resist infection by any forgein invaders (non-self)
The Immune System
Is an interacting set of specialized cells and proteins designed to identify and destroy foreign
invaders and abnormal substances befor they can damage the body.
Function of immune system:
Defends body against foreign invaders
Can distinguish between self (normal component of the body) and non-self (foreign component)
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Immune system divided into:
Non-specific immune System
(innate or natural)
Specific immune System
(adaptive or acquired)
innate Immune system
Acquired Immune system
There is immediate maximal
response
There is a lag time between
exposure and maximal
response
Non-specific
specific
Exposure results in no
immunologic memory
Exposure results in
immunologic memory
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Innate defense against infection
• All animals have some form of innate immunity (first line of defense)
• Innate immune response are defenses that act the same whether or not an invader has
been encountered.
1.Invertebrate immune system:
Invertebrates rely only on the innate immunity for example:
Insects :
a)have an exoskeleton, which is a dry barrier that keeps out of bacteria and viruses.
b)Pathogens that breach that external defense confront a set of internal defense like: low
pH and secretion of lysozyme (digest cell wall of many bacteria)
c)Circulating insect immune cells are capable of phagocytosis, engulfing and destroying
foreign invaders
d)Insects innate immune system also includes recognition proteins that bind to molecules
found only on the outside of the bacteria, fungi and other pathogens
e)Recognition of the invading microbes triggers the production of antimicrobial peptides
that bring about the destruction of the invaders.
2- Vertebrate immune system
In vertebrates, innate immune system coexists with the more recently evolved system of
acquired immune system
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In mammals: innate immune System (natural) consists of:
A- Anatomic Barriers (skin and mucous
membrane)
B- Cellular response
- Phagocytosis (phagocytes): Ingestion and destruction of foreign particles
by microphages and macrophages (type of white blood cells
- Complement system
- NK (natural killer) (another types of WBC, are not phagocytes, they attack
cancer cells and virus –infected cells by releasing chemicals that promote
programmed cell death)
-mast cells
- inflammatory reaction
C- Soluble factors
include proteins that either attack microbes directly or impede their reproduction. like
interferons which are proteins produced by virus-infected cells that help other cells to
resist viruses (stimulating the production of antiviral proteins that block viral
reproduction)
A-The first lines of defense (Anatomical barriers ):
The natural barriers
1- the skin
2- mucous membranes
That protect organ system open to the external environment such as the digestive,
respiratory, reproductive system and urinary system
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Component
Functions
Skin and mucous membranes – mechanical factors
Intact skin
Mucous membranes
Forms a physical barrier to the entrance of microbes.
Inhibit the entrance of many microbes, but not as effective as intact skin.
Mucus
Traps microbes in respiratory and digestive tracts.
Hairs (nostril hairs)
Filter incoming air from microbes and dust in nose.
Cilia (on cells lining the respiratory
tract)
Tear ducts
Saliva
Epiglottis
Urine
Together with mucus, trap and remove microbes and dust from upper
respiratory tract.
Tears dilute and wash away irritating substances and microbes.
Washes microbes from surfaces of teeth and mucous membranes of mouth.
Prevents microbes and dust from entering trachea.
Washes microbes from urethra.
Skin and mucous membranes – chemical factors
Gastric juice
Acid pH of skin
Unsaturated fatty acids
Defensins (low molecular weight
proteins)
Surfactants
Lysozyme and phospholipase
Destroys bacteria and most toxins in stomach.
Discourages growth of many microbes.
Antibacterial substance in sebum (sweat).
found in the lung and gastrointestinal tract have antimicrobial activity.
in the lung (substances that promote phagocytosis of particles by phagocytic
cells).
found in tears, saliva and nasal secretions can breakdown the cell wall of
bacteria and destabilize bacterial membranes (Antimicrobial substance)
Skin and mucous membranes – biological factors
The normal flora
On the skin and in the gastrointestinal tract can prevent the colonization of
pathogenic bacteria
by secreting
toxic substances or by competing with
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pathogenic bacteria for nutrients or attachment to cell surfaces.
2. Second line of defense (Cellular barrirs)
• Microbes that breach a mammal’s external barriers are confronted by innate defense
cells (second line of defense or cellular response).
• These are classified as white blood cells (Phagocytes).
• They are found in the interstitial fluid as well as blood vessels
1. Phagocytes:
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They are produced throughout life by the bone marrow. They are stored there before
being distributed around the body in the blood.
• Neutrophils
are a kind of phagocyte and form about 60% of the white cells in the blood. They
travel throughout the body often leaving the blood by squeezing through the walls of
capillaries to the tissues. During an infection they are releases in large numbers from
their stores but they are short-lived cells.
• Macrophages
are also phagocytes but are larger than neutrophils and tend to be found in organs, such
as the lungs, liver, spleen, kidney and lymph-nodes and through the interstitial fluid
rather than remaining in the blood. they leave the bone marrow and travel in the blood
as monocytes, which develop into macrophages once they leave the blood and settle in
the organs, removing any foreign matter found there (eating any bacteria and virus infected cells they encounter).
• Macrophages are long-lived cells and play a crucial role initiating immune
responses since they do not destroy pathogens completely, but cut them up to
display antigens that can be recognized by lymphocytes.
• Conclusion
Macrophages : very large white cells that can move around body, or remain in certain
tissues. Long lived, act as scavengers
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ingest bacteria, viruses, dead cells, dust
most circulate in the blood, lymph and extracellular fluid
they are attracted to the site of infection by chemicals given off by dying cells
after ingesting a foreign invader, they “wear” pieces of it called antigens on their cell
membrane receptors – this tells other types of immune system cells what to look for
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Phases of phagocytosis:
1. Chemotaxis and adherence of microbe to phagocyte:
when pathogens invade the body and cause an infection, some of the cells under
attack respond by releasing chemicals such as histamine. These with any chemicals
released by the pathogens themselves, attract passin phagocytes to the site. The
phagocytes destroy the pathogens by phagocytosis .
2. Attachments and uptake (Ingestion of microbes by phagocyte):
The phagocytes move towards the pathogens, which may be clustered together and
covered in antibodies. This further stimulates the phagocytes to attack them. This is
because phagocytes have receptor proteins on their surfaces that recognize antibody
molecules and attach to them.
3. Digestion:
formation of phagosome, fusion of phagosome with lysosyme to form
phagolysosome, digestion of the ingested microbes and the formation of residual
body.
4. After ingesting a foreign invader,
they “wear” pieces of it called antigens (antigenic determinant) on their cell
membrane receptors (Major Histocompatibility Complex (MHC)– in this case it
named Antigen Presenting Cell (APC) this tells other types of immune system
cells what to look for
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2. Second line of innate defence cont.
Complement:
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Complement is not a cell but a group of proteins
set of about 30 different kinds of proteins that circulate in an inactive form in the
blood
These proteins can act together (in complement) with other defense mechanisms
made in the liver
Activated by infection: Substances on the surfaces of many microbes trigger a cascade
of steps that activate the complement system, leading to the lyses (bursting) of the
invaders, activated complements:
– help to recruit phagocytes to site of inflammation and activate them
– bind to receptors on phagocytes, helping to remove agent of infection
– form pores in the invader or infected cell’s membrane (like the NKs do)
– activate mast cells to release histamine and other factors
– Certain complement proteins also help trigger the inflammatory responses
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Dr. Sanaa Tork
Natural killer cells (NK cells)
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instead of attacking the invaders, they attack the
body’s own cells that have become infected by
viruses
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they also attack potential cancer cells, often before
they form tumors
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they bind to cells using an antibody “bridge”, then
kill it by secreting a chemical (perforin) that makes
holes in the cell membrane of the target cell. With
enough holes, the cell will die, because water
rushing inside the cell will induce osmotic
swelling, and an influx of calcium may trigger
apoptosis.
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Mast cells
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found in tissues (connective tissues and mucous
membranes) like the skin, near blood vessels.
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are activated after antigen binds to a specific type of
antibody called IgE that is attached to receptors on the mast
cell.
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activated mast cells release substances that contribute to
inflammation, such as histamine.
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mast cells are important in allergic responses but are also
part of the innate immune response, helping to protect from
infection.
Soluble factors
• Interferon
Interferon is a protein produced by virus-infected cells that inhibits the
synthesis of viral proteins, leading to decreased viral replication. It also can
cause apoptosis of virus-infected cells.
Acute phase proteins
– proteins in the plasma that increase during infection and inflammation
– can be used diagnostically to give an indication of acute inflammation
– An example of an acute phase protein is C-reactive protein, which fixes
complement.
INNATE IMMUNE SYSTEM
Cellular response
1- Inflammatory response:
1- Is a major component of our innate immune system
2- any damage to tissue, whether caused by microorganisms or by physical injury
triggers this response
3- the injury area become red, worm and swollen this reaction is inflammation
Inflammation
The damage cells release chemical alarm signals such as histamine, which is a vasodilator.
This causes localised swelling, redness, heat, pain. Can also cause high temperature, brings
white cells to the area of infection
The major results of inflammatory response are :
1- to disinfect and clean injury area
2- also helps prevent the spread of infection to surrounding tissues
3- Clotting proteins present in blood plasma pass into the interstitial fluid during
inflammation, along with platelets form local clots that help seal off the infected
region and allow healing to begin.
Events that make up the inflammatory response in case where a pin
has broken the skin, allowing infection by bacteria:
1- the damage cells soon release chemical alarm signals like histamine and prostaglandin
2- the chemicals spark the mobilization of various defenses, histamine and prostaglandin
induces neighboring blood vessels to dilate and become leakier. Blood flow to the
damage area increases and blood plasma pass out of the leaky vessels into the
interstitial fluid of the affected tissues. Other chemicals (some of complement system)
attract phagocyte to the affected area. The phagocytes squeeze between the cells of the
blood vessels wall. The local increase in blood flow, fluid and cells produces the
redness, heat, and swelling characteristic of inflammation.
3- the phagocytes engulf bacteria and the remains of damage body cells, many blood cells
die in the process, their remains are also engulfed and digested. The pus that often
accumulates at the site of an infection consists mainly of dead white cells and fluid that
has leaked from the capillaries during the inflammatory response.
4- the inflammatory response may be localized or widespread (systemic), some bacteria
or protozoans get into the blood or release toxins that carried throughout the body in the
bloodstream. Another response to systemic infection is fever, an abnormally high body
temperature.
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Dr. Sanaa Tork
The release of histamine and prostaglandin causes local vessel dilation resulting in:
• increased capillary permeability
• increased blood flow  more WBCs to site  redness and warmth
• phagocytes move out of vessels into intracellular fluid (ICF)
• edema (swelling) due to fluids seeping from capillaries
Fevers have both positive and negative effects on infection and body functions
POSITIVE (moderate fever)
• indicate a reaction to infection
• stimulate phagocytosis and hasten tissue
repair
• slow bacterial growth
– increases body temperature beyond
the tolerance of some bacteria
– decreases blood iron levels
(Lactoferrin and Transferrin)
NEGATIVE
• extreme heat  enzyme denaturation
and interruption of normal biochemical
reactions
> 39° C (103°F) is dangerous
> 41°C (105°F) could be fatal and
requires medical attention, leading
to a condition called septic shock
which characterized by very high
fever and low blood pressure
Immune system
Acquired
immune system
Innate
immune system
Humoral
(antibody mediated)
Cellular
(Cell mediated)
immune response
immune response
B-lymphocytes
T-lymphocytes
1st line of
defense
Skin
Mucous
membrane
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2nd line of
defense
Inflammation
Phagocytosis
Complement
Interferons
Natural killers
The acquired immune system
1. When the innate immune response fails to ward off a pathogen, the acquired immune
response provides a second line of defenses.
2. Acquired immunity found only on vertebrates, is a set of defenses that are activated only
after exposure to pathogens.
3. Once activated , the acquired immune response provides a strong defense against
pathogens that is highly specific (acts against one infectious agent but not another)
4. Acquired immune response has a remarkable memory (it can remember antigen it has
encountered before and react against them)
5. Can amplify certain innate responses such as inflammation and the complement system
6. acquired immunity is usually obtained by natural exposure to antigens but it can also be
achieved by vaccination (vaccine is composed of a harmless variant or part of a diseasecausing microbe such as an inactived bacterial toxin, a dead or weakened microbe, or a
pieces of a microbe)
7. Vaccination also known as immunization, because vaccine stimulates the immune system
to mount specific defense against this harmless antigen, if it is exposed to the actual
microbe, our immune system will respond quickly and effectively
8. For this reason the widespread of vaccination of children has virtually eliminated some
viral diseases such as polio, smallpox and measles.
Induced Immunity (acquired immune response)
Passive immunity
Active immunity
1. An individual is given antibodies by another
(received of premade antibodies)
2. Short-term resistance (weeks- 6months)
Production of a person’s own antibodies.
Long lasting
Natural Active
Artificial Active
When pathogen enters
body in the normal
way, we make
antibodies
• Vaccination – usually
contains a safe antigen
from the pathogen.
• Person makes antibodies
without becoming ill
3.Passive immunity is temporary because
the recipient’s immune system is not
stimulated by antigen
Natural Passive
Artificial Passive
•Fetus obtains
• Gamma globulin
antibodies from its
injection
mother’s bloodstream.
• extremely fast, but short
•Baby receives
lived
antibodies in breast milk • (e.g. snake venom, which
controlled by injecting
the victim with
antivenin)
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•Antigens (antibody generating) – foreign molecules that stimulate an acquired immune
response which responds with an increase in the number cells that either attack the invader
directly or produce immune proteins called antibody
•Most antigens are proteins or large polysaccharides on the surface of foreign invaders such
as protein coat molecules of viruses, parts of the capsules and cell walls of bacteria and ;
macromolecules on the surface cells of other kinds of organisms such as protozoan and
parasitic worms or antigenic molecules on blood cells or tissue cells from other individuals
or found dissolved in body fluids like bacterial toxins and bee venom.
•Antibodies (immunoglogulins) – proteins found in the blood plasma that attached to one
particular kind of antigen and helps counter its effects It produced by lymphocytes in
response to antigens
•Antibody usually recognizes and binds to a small surface-exposed region of an antigen that
binds to antigen-binding site of antibody called antigenic determinant or epitope
•Paratope or antigen-binding site: a specific region on the antibody molecule, recognize
an antigenic determinant by the fact that the binding site and antigenic determinant have
complementary shapes (like lock and key)
Paratope
• An antigen usually has several different determinants, so different antibodies can bind
to the same antigen
• The immune system’s ability to defend against an enormous variety of antigens depends
on a process known as clonal selection:
(one particular antigen interacts only with the tiny fraction of lymphocytes bearing
receptors specific to that antigen, once activated by the antigen, these few selected cell
proliferate, forming a clone (genetically identical population) of thousands of cells all
specific for the stimulating antigen)
this antigen-driven cloning of lymphocytes- clonal selection is the vital step in the
acquired immune response against infection
• Steps of clonal selection
1- The first time an antigen enters the body and is
swept into the lymph node
2- antigenic determinants on its surface bind to the
few B cells that have complementary receptors
3-the selected cell is activated , grow,
divided and differentiates into two distinct
types of cells (memory cell and plasma cell
(effector cell))
4- each plasma cell secrets antibody
molecules (as many as 2000 copies of its
antibody per second) each plasma cell
require large amounts of endoplasmic
reticulum, the secreted antibodies circulate
in the blood and lymphatic fluid,
contributing to the humoral immune
response. Each effector cell lasts only 4-5
days before dying off
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• Two groups of cells produced by the activated B cells:
1. Plasma cell which are highly effective and combating infection, it lasts only 4-5 days
(short lived)
2. Memory cell which differ from effector cells in both appearance and function, it lasts
for decades in the lymph nodes, activated by a second exposure to the antigen
These steps show the primary immune response this phase occurs when lymphocytes are
exposed to an antigen for the first time.
When the produced memory cells activated by the second exposure to the same antigen,
they initiate the secondary immune response, this response is faster and stronger than the
first, the selected memory cells multiply quickly, producing a large second clone of
lymphocytes, produces very high levels of antibodies than the first response, effector
cells are often more effective against antigen than those produced during the primary
response
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Antibody Molecules
1. Antibodies are the weapons of the humoral immune response
2. Each antibody molecule is made of four peptide chains, two identical heavy chains (give
the molecule its Y shape) and two identical light chains, each light chain bounded to one
of the heavy chain, at the fork of the Y bonds the two heavy chains
3. Each of the four chains of the molecule has a c (constant) region and a V (variant) region.
At the tip of each arm of the Y, a pair of V regions forms an antigen-binding site (a region
of a molecule responsible for the antibody’s recognition and binding function)
4. A huge variety in the three-dimensional shapes of the binding sites of different antibody
molecules arises from a similarly large variety in the amino acid sequences in the V
regions . This structural variety account for the diversity of lymphocytes and given the
humoral immune system the ability to react to any kind of antigen.
Region binds to Complement
Region binds to Macrophage
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The tail of the antibody molecule, formed by the constant regions of the heavy
chains, helps mediate the disposal of the bound antigen.
Antibodies with different kinds of heavy chains C regions are grouped into
different classes.
Human and other mammals have five major classes of antibodies called : IgA, IgD,
IgE, IgG, IgM
Antibodies inactive antigen by:
1.agglutination of microbes :makes pathogens
clump together, enfances phagocytosis.
2. Neutralization (antitoxins) neutralize the toxins
produced by bacteria, blocks viral binding sites,
coats bacteria.
3.Activation of complement system (leads to cell
lysis) digests the bacterial membrane, killing
the bacterium
4.opsonisation (precipitation of dissolved
antigens) coats the pathogen in protein that
identifies them as foreign cells
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Lymphocytes:
• Lymphocytes are white blood cells, smaller than phagocytes. They have a large nucleus
that fills most of the cell., like all blood cells, lymphocytes originate from stem cells
before birth in bone marrow, there are two types of lymphocyte
1. B lymphocytes (B cells) continue developing in the bone marrow until they are mature
and then spread throughout the body concentrating in lymph nodes and the spleen.
2. T lymphocytes ( T cells ) leave the bone marrow and collect in the thymus where they
mature. Only mature lymphocytes can carry out immune responses. Both types of
lymphocytes are responsible for the acquired immune system
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During the maturation process :
• many different types of B and T lymphocyte develop, Perhaps many millions, certain
genes in the lymphocyte cell are turn on , this leads the cell to synthesize molecules of
specific protein, incorporated into the plasma membrane. The molecules are antigen
receptors, capable of binding one specific type of antigen, each type is specialized to
respond to one antigen, giving the immune system as a whole the ability to respond to
almost any type of pathogen that enters the body.
• Each T and B cells has about 100 000 antigen receptors on a single cell and all the
receptors on a single cell are identical- they all recognize the same antigen
• In case of B cells , the receptors are almost identical to the particular antibody that the B
cell will secrete.
• When mature, both B and T cells circulate between the blood and the lymph. This
ensures that they are distributed throughout the body so that they come into contact with
any pathogens and with each other.
• Acquired immune responses depend on B and T cells interacting with each other to give
an effective defense.
• When a B and T cell within a lymphatic organ first confronts the specific antigen it is
programmed to recognize, it differentiates further and becomes a fully mature component
of the immune system.
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Two types of acquired immune response:
1- The humoral immune response:
- involves the secretion of antibodies by B cell into the blood and lymph,
- the humoral defense defends primary against bacteria and viruses present in body fluids
- this defense can be passively transferred by injecting blood plasma (containing
antibodies) from an immune individual into a non immune individual
2- The cell- mediated immune response:
- produced by T cells
- this defensive system results from the action of defensive cells rather than the
defensive proteins of the humoral response
-certain T cells attack body cells that are infected with bacteria or viruses, others
function indirectly by promoting phagocytosis by other blood cells and by
stimulating B cells to produce antibodies
Thus T cells play a part in both the cell- mediated and humoral immune response
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• Briefly, some Tcells (TH) coordinate the immune response, stimulating B
cells to divide and then secrete antibodies into the blood; these antibodies
destroy the antigenic pathogens. Other Tcells (NK) seek out and kill any of
the body’s own cells that are infected with pathogens. To do this they must
make direct contact with infected cells.
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T helper cells stimulate humoral and cell-mediated immune
responses
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Helper T cells interact with others white blood cells –including macrophages, B cells,
and other types of immune cells-that function as antigen –presenting cells.
All the cell mediate immune response and much of the humoral immune response
depend on the precise interaction of antigen-presenting cells and helper T cells.
The interaction activate other cells of the immune system
The antigen presenting cells presents a foreign antigen to helper T cell.
The ability of a helper T cell to recognize a unique self-nonself complex on an antigen
– presenting cell depends on the receptors embedded in the T cell’s plasma membrane.
T cell receptor actually has two binding sites: one for antigen and one self protein. The
two binding sites enable T cell receptor to recognize the overall shape of a self-nonself
complex on an antigen –presenting cell.
The immune response is highly specific because the receptors on each helper T cell
can bind only one kind of self – nonself complex on an antigen-presenting cell.
The binding of T cell receptor to self –nonself complex activates the helper T cell
Other kinds of the signals can enhance this activation such as interleukin-1 which
diffuse to the T helper cell and stimulate it.
Activated helper T cell promote the immune response in several ways, being the
secretion of addational stimulatory proteins such as:
interleukin-2 which make T helper grow and divide, producing both memory cells and
qdditional active helper T cells. It also activate B cells , thus stimulating the humoral
immune response, and stimulates the activity of cytotoxic T cells
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B-lymphocytes
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The antibody- producing B-cells of the humoral response make up one arm of the
acquired immune response network
Humoral defense system identifies and helps destroy invaders that are in our blood,
lymph or interstitial fluid (outside our body cell)
B cell respond to free antigens present on the surface of the body fluids, T cells
respond only to antigens present on the surface of the body’s own cells
B lymphocytes make antibodies = immunoglobulins
1000s of different B cells, each recognises a
different antigen on the surface of a macrophage.
Each antigen stimulates production of a single
specific antibody
B cells (along with T cells) come in contact with antigen.
They are stimulated (by T cells) to produce many clones,
plasma cells, which make antibodies.
Memory B cells – faster, more sensitive reaction
= secondary response
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How B-cells work…
Pathogen (e.g. bacteria, virus)
Macrophage
B-cells
Each recognise
a different
antigen. The
correct one
develops into…
Plasma cells
Clones of the
correct B-cell,
which produce
antibodies
Macrophage
Phagocytoses pathogen
and displays antigens on
surface
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1st meeting a pathogen, this
process takes 10-14 days
Memory B cell= subesquent
meetings, takes about 5 days
T lymphocytes
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Mature T cells have specific cell surface receptors called T cell receptors.
T cells are activated when they encounter this antigen in contact with another host
cell.
Sometimes this is a macrophage that has engulfed a pathogen and cut it up to expose
the pathogen's surface molecules or it may be a body cell that has been invaded by a
pathogen and is similarly displaying the antigen on its plasma membrane as a kind of
“signal” . Those T cells that have matching receptors respond to the antigen by
dividing.
• There are two main types of T cell:
1. T helper cells;
2. Killer T cells .
when T helper cells are activated they release hormone – like cytokines that
stimulate appropriate B cells to divide, develop into plasma cells and secrete
antibodies. Some T helper cells secret cytokines that stimulate macrophges to carry
out phagocytosis more vigorously. Killer T cells search the body for cells that have
become invaded by pathogens and are displaying foreign antigens from the pathogens
on their plasma membranes. Killer T cells recognise the antigens, attach themselves to
the surface of infected cells and secrete toxic substances, killing the body cells and the
pathogens inside.
In addition to the helper cells and killer cells, memory T cells are produced which
remain in the body and become active very quickly during the secondary response to
antigens.
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T-lymphocytes
Mature in Thymus, which is most active just before and after birth.
The thymus starts to shrink during puberty.
Helper T-Cells
•Recognise antigens on
surface of leukocytes,
especially macrophages
•Enlagre and form a
clone of T-helper cells
•Secrete interferon and
cytokines which
stimulate B-cells and
stimulate killer -cells
•Can be infected by HIV
Killer T-Cells
Also called cytotoxic
•Destroy abnormal body
cells, e.g. virus infected
or cancer cells
•Stimulated by cytokines
(THcells)
•Release perforin, which
forms pores in target
cells. This allows water
and ions in = lysis
Suppressor T-Cells
•Control the
immune system
when the antigen
/pathogen has
been destroyed
•Only recently
discovered so
little is known
about them
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Memory T-Cells
•Can survive a long time
and give lifelong
immunity from
infection
•Can stimulate memory
B-cells to produce
antibodies
•Can trigger production
of killer T cells
Abnormal cell e.g
cancer cell, infected cell
Killer T-cell
recognises antigen
How T-cells work…
X
Antigen
Clones of killer T-cell
attach to antigen
Normal cell
X
Killer T-cells release
perforin pores
X
Helper T-cell stimulates
correct killer T-cell to
multiply
Helper T-cell also
stimulates B-cells
to make antibodies
Suppressor T-cells
turn off immune
response
Abnormal cell gains
water, swells and
bursts
Memory Tcells stay in
circulation
Dr. Sanaa Tork
Dr. Sanaa Tork
Duration of immunity
Memory B-cells circulate for a long time. If the same pathogen infects the
body again, these B-cells can produce large amounts of specific antibody
very quickly. This is why you usually don’t suffer from the same infection
twice.
Memory T-cells survive a long time and trigger an immune response
Immune disorders
•Sometimes the body produces antibodies against its own tissues e.g. autoimmune
diseases e.g. rhumatoid arthritis, Crohn’s disease, SCID (bubble boy disease),
Asthma
In Lupus, for example, B cells make antibodies against a wide range of self molecules
such as histones and DNA released by the normal breakdown of body cells.
Lupus is characterized by skin rashes, fever, arthritis and kidney mal-function
In rheumatoid arthritis is another antibody-mediated autoimmune disease, it leads
to damage and painful inflammation of the cartilage and bone of joints
In multiple sclerosis, T cells react against the myelin sheath , peoples with MS has a
number of serious neurological abnormalities
•In Crohn disease, a chronic inflammation of the digestive tract, may be caused by
an autoimmune reaction against normal flora that inhabit the intestinal tract.
Dr. Sanaa Tork
Immune deficiency diseases: here immune deficient people lack one or more of the
component of the immune system this makes them susceptible to frequent and
recurrent infections.
In rare congenital disease severe combined immunodeficiency (SCID) both T and B
cells are inactive or absent, people with SCID are sensitive to even minor infection.
•Allergies occur when the body reacts to antigens in our surrounding e.g.
peanut
•Antigens that cause allergies are called allergins
•Tumours – in most cases the body recognises tumours as being bad, because they
express abnormal molecules on the cell surface. However sometimes the body doesn’t
notice and cancers can develop
Dr. Sanaa Tork