TRANSMISSION OF PATHOGENS

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TRANSMISSION OF PATHOGENS

• Infective agents can be transmitted from one host
  to another by
• A VECTOR
• A carrying vector eg rats fleas
• An injecting vector eg mosquito malaria
• direct contact
• Droplet infection in air breathed or sneezed out
• Sexual contact
• Contaminated food or water
• Injecting with infected needle syringe

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Transmission of Disease

• Diseases can be transmitted in three broadly
  different ways
• Contact transmission
• Vehicle transmission
• Vector transmission

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Vector Transmission

• Many pathogens have more than one host. An
  intermediate host may transmit the pathogen to
  its primary host.
• Bites from a variety of animals can introduce
  pathogens.

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Contact Transmission

• Pathogens may be spread by contact with other
  infected humans or animals.

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Vehicle Transmission

• Disease may be transmitted through a medium such
  as blood, water, food, or air.

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Bacteria and Disease

• Of the many species ofbacteria that exist in
  theworld, relatively few arepathogenic.
• Most bacteria form partof the normal microflora
  found on healthy humans.
• Bacteria infect a host in order to exploit the
  food potential of the hosts body tissues. The
  fact that this exploitation causes disease is not
  in the interest of the bacteria a healthy host
  is better than a sick one.

Photo CDC/Dr Mike Miller.
Human vaginal epithelial cell
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The Bodys Defenses

• If microorganisms never encountered resistance
  from our defenses, we would be constantly ill and
  would eventually die of various diseases.

Nonspecific Defense Mechanisms Nonspecific Defense Mechanisms Specific Defense Mechanisms
1st line of defense 2nd line of defense 3rd line of defense
Intact skin Mucous membranes and their secretions Phagocytic white blood cells Inflammation and fever Antimicrobial substances Specialized lymphocytes(B-cells and T-cells) Antibodies
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Non specific defences

• These defences do not differentiate between any
  disease causing agents. They stop all things from
  entering the body.

• First line of Defence
• Enzymes in mucus, tears, gut
• Skin
• Sweat (contains acid)
• Ciliated epithelium
• Histamines

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Eyes Tears wash out pathogens and also contain an enzyme that can kill bacteria. MouthFriendly bacteria help to prevent the growth of harmful pathogens.Saliva cleans and removes bacteria. Lungs Mucus in the lungs traps bacteria and fungal spores. Tiny hairs, called cilia, move the mucus to the back of the throat where it is swallowed.
NoseMucus traps pathogens which are then swallowed or blown out in coughs and sneezes. SkinThe outer layer of skin is dead and difficult for pathogens to grow on or penetrate.Cuts allow pathogens to gain entry to the body. Reproductive systemSlightly acid conditions in the vagina and urethra help to stop the growth of pathogens. MouthFriendly bacteria help to prevent the growth of harmful pathogens.Saliva cleans and removes bacteria. Lungs Mucus in the lungs traps bacteria and fungal spores. Tiny hairs, called cilia, move the mucus to the back of the throat where it is swallowed.
NoseMucus traps pathogens which are then swallowed or blown out in coughs and sneezes. SkinThe outer layer of skin is dead and difficult for pathogens to grow on or penetrate.Cuts allow pathogens to gain entry to the body. Reproductive systemSlightly acid conditions in the vagina and urethra help to stop the growth of pathogens. StomachAcid helps to sterilise the food. Large intestineFriendly bacteria help to stop the growth of harmful pathogens.Faeces contains over 30 live bacteria.




                                   
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THE NON-SPECIFIC IMMUNE RESPONSE
Second line of Defence
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Defence against disease (2nd Line)

• Cell-mediated defences involving phagocytic cells
  appear to have been present early in the
  evolution of animals. Most organisms are able to
  distinguish self from not self.

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Recognising SELF

• The bodies immune system has the ability to
  recognise self from non-self. This is
  possible because all our cells have specific
  protein markers on their surface called ANTIGENS.
• Genes on chromosome number 6, called the Major
  Histocompatibility Complex (MHC), code for the
  production of these self MHC antigens

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Distinguishing Self

• The human immune system achievesself-recognition
  through the majorhistocompatibility complex
  (MHC).
• The MHC is a cluster of tightly linked genes on
  chromosome 6 in humans.
• These genes code for protein molecules (MHC
  antigens) which are attached to the surface of
  body cells.

Location of genes on chromosome 6 for producing
the HLA antigens
Class I HLA
Class II HLA
HLA surface proteins (antigens) provide a
chemical signature that allows the immune system
to recognize the bodys own cells
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MHC

• The MHC antigens are used by the immune system to
  recognize its own and foreign material.
• Class I MHC antigens are located onthe surface
  of virtually all human cells.
• Class II MHC antigens are restricted
  tomacrophages and B-lymphocytes

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Second Line of Defence(Internal)

• These include
• Phagocytes Lymphocytes which are White
  blood cells
• Proteins called Antibodies which destroy
  pathogens
• Complement system which is large blood proteins
  that destroy bacteria
• Interferon (proteins) which are produced by
  virus infected cells and interfere with viral
  reproduction
• Inflammation

• Once a foreign material enters the body the
  second line of defense comes into play.

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Blood Cells
White Blood Cells Phagocytes Neutrophil

Macrophages Lymphocytes
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Phagocytes

• Produced throughout life by the bone marrow.
• Scavengers remove dead cells and
  microorganisms.
• Phagocytes are white blood cells that ingest
  microbes and digest them by phagocytosis.

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The Action of Phagocytes
Microbes
Nucleus
Phagosome
Lysosome
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Phagocytosis
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Neutrophils

• 60 of WBCs
• Patrol tissues as they squeeze out of the
  capillaries.
• Large numbers are released during infections
• Short lived die after digesting bacteria
• Dead neutrophils make up a large proportion of
  puss.

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Monocytes

• Monocytes and neutrophils share the same stem
  cell. (Monocytes are to macrophages what Bruce
  Wayne is to Batman.) They are produced by the
  marrow, circulate for five to eight days, and
  then enter the tissues where they are
  mysteriously transformed into macrophages. Here
  they serve as the welcome wagon for any outside
  invaders and are capable of "processing" foreign
  antigens and "presenting" them to the
  immunocompetent lymphocytes. They are also
  capable of the more brutal activity of
  phagocytosis

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Eosinophils

• Eosinophils respond to chemotaxis, substances
  released by bacteria and components of the
  complement system and can perform phagocytosis.
  They are often seen at the site of invasive
  parasitic infestations and allergic (immediate
  hypersensitivity) responses. Individuals with
  chronic allergic conditions (such as atopic
  rhinitis or extrinsic asthma) typically have
  elevated circulating eosinophil count.

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Lymphocytes

• When activated by whatever means, lymphocytes can
  become very large. Although such cells are
  classically associated with viral infection, they
  may also be seen in bacterial and other
  infections and in allergic conditions.

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Platelets

• Platelets are small fragments of cells found in
  blood and their main function is involved in the
  blood clotting process.

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Macrophages

• Larger than neutrophils.
• Found in the organs, not the blood.
• Made in bone marrow as monocytes, called
  macrophages once they reach organs.
• Long lived
• Initiate immune responses as they display
  antigens from the pathogens to the lymphocytes.

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Defensive molecules

• Cytokines are an important group of signalling
  molecules that coordinate many aspects of our
  immune responses. They are small glycoproteins
  released by body cells as a means of
  communication with the immune system.
• Cytokines indicate the presence of damage or a
  potentially dangerous invader.

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• Interferons are a class of cytokines. They are
  produced by most virus-infected cells during
  viral invasion and are also secreted by activated
  T cells.
• Their production and secretion is triggered by
  the presence of double-stranded RNA, which does
  not occur in uninfected cells.
• Interferons are very active in interfering with
  virus replication in cells.

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Complement system

• The complement system is a very complex group of
  20 serum proteins which is activated in a cascade
  fashion.
• Three different pathways involved in complement
  activation.
• The first recognizes antigen-antibody complexes,
• the second spontaneously activates on contact
  with pathogenic cell surfaces,
• the third recognizes mannose sugars, which tend
  to appear only on pathogenic cell surfaces.

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• A cascade of protein activity follows complement
  activation this cascade can result in a variety
  of effects including phagocytosis of the
  pathogen, destruction of the pathogen by
  formation and activation of the membrane attack
  complex, and inflammation.

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The organs of your immune system are positioned
throughout your body. They are called lymphoid
organs because they are home to lymphocytes--the
white blood cells that are key operatives of the
immune system. Within these organs, the
lymphocytes grow, develop, and are deployed. Bone
marrow, the soft tissue in the hollow center of
bones, is the ultimate source of all blood cells,
including the immune cells. The thymus is an
organ that lies behind the breastbone
lymphocytes known as T lymphocytes, or just T
cells, mature there. The spleen is a flattened
organ at the upper left of the abdomen. Like the
lymph nodes, the spleen contains specialized
compartments where immune cells gather and
confront antigens.
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The Third Line of Defense

• Specific resistance is a third line of defense.
  It forms the immune response and targets specific
  pathogens.
• Specialized cells of the immune system, called
  lymphocytes are
• B-cells produce specific proteins called
  antibodies, which are produced against specific
  antigens.
• T-cells target pathogens directly.

The 2nd line of defense
The 3rd line of defense
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Specific Immunity

• This is the third line of defense and has the
  ability to remember a previously encountered
  organisms so as to attack them.
• This includes

• Immune responses
• Specificity that is they act on certain
  foreign objects
• Memory this is where the system remembers the
  foreign object.

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Plant immunity

• To defend against parasites plants use
  encapsulation, a vast array of chemical defences
  including antibiotics, enzymes and hormones that
  disrupt the function of parasites. They also
  allow rapid death of tissue under attack.

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Immune system of mammals

• The immune response of mammals involves
• Humoral immunity antibodies are released by B
  cells
• Cell mediated immunity
• - active destruction by T cells

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SPECIFIC IMMUNITY

• Two main groups of LYMPHOCYTES are involved in
  specific immunity. All lymphocytes are made in
  the bone marrow. Some mature in the bone marrow
  to become B cells others leave early to mature in
  the Thymus, they become T cells.

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Specific Immunity
This is the third line of defense and has the
ability to remember a previously encountered
organisms so as to attack them. This includes

• Immune responses
• Specificity that is they act on certain
  foreign objects
• Memory this is where the system remembers the
  foreign object.

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White blood cells (leukocytes)

• Are a diverse group of blood cells, all are-
• Manufactured in the bone marrow
• Possess a nucleus
• Play a role in response to pathogens and/or
  foreign material
• Capable of independent movement
• Many have a role in non-specific defences.
  Lymphocytes are important in specific defences.

Handout
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Lymphocytes

• Produce antibodies
• B-cells mature in bone marrow then concentrate in
  lymph nodes and spleen
• T-cells mature in thymus
• B and T cells mature then circulate in the blood
  and lymph
• Circulation ensures they come into contact with
  pathogens and each other

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Humoral immune response

• Humoral means in the body fluids (blood and
  extracellular)
• B cells are lymphocytes that produce large
  quantities of antibodies when stimulated by
  particular antigens. This is the humoral immune
  response.
• B cells are made in the bone marrow and spleen.
• B cells have immunoglobulins (a protein that
  identify antigens) on their surface.
• Each B cell identifies one kind of antigen only.
• When B cells identify an antigen, it replicates
  rapidly to produce large numbers of special cells
  called PLASMA cells.

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Humoral Immunity
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BCells

• B-cells (also called B-lymphocytes) originate and
  mature in the bone marrow of the long bones (e.g.
  the femur). They migrate from the bone marrow to
  the lymphatic organs.
• B-cells defend against
• Bacteria and viruses outside the cell
• Toxins produced by bacteria (free antigens)
• Each B-cell can produce antibodies against only
  one specific antigen.
• A mature B-cell may carry as many as 100 000
  antibody molecules embedded in its surface
  membrane.

B-cell (B-lymphocyte)
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B -Lymphocytes

• There are approx 10 million different
  B-lymphocytes, each of which make a different
  antibody.
• The huge variety is caused by genes coding for
  antibodies changing slightly during development.
• There are a small group of clones of each type of
  B-lymphocyte

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B -Lymphocytes

• At the clone stage antibodies do not leave the
  B-cells.
• The antibodies are embedded in the plasma
  membrane of the cell and are called antibody
  receptors.
• When the receptors in the membrane recognise an
  antigen on the surface of the pathogen the B-cell
  divides rapidly.
• The antigens are presented to the B-cells by
  macrophages

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BCell Differentiation

• B-cells differentiate into two kinds of cells
• Memory cellsWhen these cells encounter the same
  antigen again (even years or decades after the
  initial infection), they rapidly differentiate
  into antibody-producing plasma cells.
• Plasma cellsThese cells secrete antibodies
  against antigens. Each plasma cell lives for only
  a few days, but can produce about 2000 antibody
  molecules per second.

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B -Lymphocytes
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B -Lymphocytes

• Some activated B cells become PLASMA CELLS these
  produce lots of antibodies, lt 1000/sec
• The antibodies travel to the blood, lymph, lining
  of gut and lungs.
• The number of plasma cells goes down after a few
  weeks
• Antibodies stay in the blood longer but
  eventually their numbers go down too.

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B -Lymphocytes

• Some activated B cells become MEMORY CELLS.
• Memory cells divide rapidly as soon as the
  antigen is reintroduced.
• There are many more memory cells than there were
  clone cells.
• When the pathogen/infection infects again it is
  destroyed before any symptoms show.

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How B-cells work
1st meeting a pathogen, this process takes 10-14
days Memory B cell subesquent meetings, takes
about 5 days
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Antibodies

• Also known as immunoglobulins
• Globular glycoproteins
• The heavy and light chains are polypeptides
• The chains are held together by disulphide
  bridges
• Each antibody has 2 identical antigen binding
  sites variable regions.
• The order of amino acids in the variable region
  determines the shape of the binding site

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Antibodies

• Antibodies are specific proteins produced by
  lymphocytes that react with particular antigen
  molecules
• Antigen substance capable of binding with
  antibody
• Antibody specific protein which binds with
  antigen

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How Antibodies work

• Some act as labels to identify
• antigens for phagocytes
• Some work as antitoxins i.e. they block toxins
  for e.g. those causing diphtheria and tetanus
• Some attach to bacterial flagella making them
  less active and easier for phagocytes to engulf
• Some cause agglutination (clumping together) of
  bacteria making them less likely to spread

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Antigens and Antibodies
Molecular model
Symbolic model

• Antibodies recognize and bind to antigens.
• Antibodies are highly specific and can help
  destroy antigens.
• Each antibody has at least two sites that can
  bind to an antigen.

Antibody
One of the two binding sites on the antibody
Antigen
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Antibody Structure
Most of an antibody molecule is made up of
constant regions which are the same for all
antibodies of the same class.
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Type Number of antigen binding sites Site of action Functions
IgG 2 Blood Tissue fluid CAN CROSS PLACENTA Increase macrophage activity Antitoxins Agglutination
IgM 10 Blood Tissue fluid Agglutination
IgA 2 or 4 Secretions (saliva, tears, small intestine, vaginal, prostate, nasal, breast milk) Stop bacteria adhering to host cells Prevents bacteria forming colonies on mucous membranes
IgE 2 Tissues Activate mast cells ? HISTAMINE Worm response
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Blood group Antigens present on the red blood cells Antibodies present in the plasma
A Contains anti-B antibodies, but no antibodies that would attack its own antigen A
B Contains anti-A antibodies, but no antibodies that wouldattack its own antigen B
AB Contains neither anti-A or anti-B antibodies
O Contains both anti-A and anti-B antibodies
antigen A
antigen B
antigens A and B
Neither antigen A nor B
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Clonal Selection Theory

• The clonal selection theory is the accepted model
  for how the immune system responds to infection
  and how certain types of B and T lymphocytes are
  selected for specific antigens invading the body.

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There are 4 parts

• Each lymphocyte has a single type of receptor
  with a unique specificity.
• Receptor occupation is required for cell
  activation.
• The differentiated effector cells derived from an
  activated lymphocyte has receptors of identical
  specificity as the parental cell.
• Those lymphocytes bearing receptors for self
  molecules will be deleted early.

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Cell mediated immune response

• T cells are responsible for cell mediated immune
  responses. They act against virus infected cells,
  cancer cells and transplanted tissue
• T cells are formed in the thymus gland from
  precursor cells made in bone marrow

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T-Cells

• T-cells originate from stem cells and mature
  after passing through the thymus gland. They
  respond only to antigenic fragments that have
  been processed and presented bound to the MHC by
  infected cells or macrophages (phagocytic cells).
  
• T-cells defend against
• Intracellular bacteria and viruses.
• Protozoa, fungi, flatworms, and roundworms.
• Cancerous cells and transplanted foreign tissue.

Molecular Immunology Foundation,
www.mifoundation.org
T-cells attacking a cancer cell
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T-Cells

• T-cells can differentiate into four specialized
  types of cell
• Helper T-cell
• Activates cytotoxic T cells and other helper T
  cells.
• Necessary for B-cell activation.
• Suppressor T-cell
• Regulates immune response by turning it off when
  no more antigen is present.
• T-cell for delayed hypersensitivity
• Causes inflammation in allergic reactions and
  rejection of tissue transplants.
• Cytotoxic (Killer) T-cell
• Destroys target cells on contact.

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Types of T cells

• T helper cells
• acts with T cytotoxic cells (Killer T cells) to
  destroy fungi, virus infected cells, cancer cells
  and transplanted tissue
• Work with B plasma cells to create antibodies
  which inactive toxins bind to bacteria, causing
  clumping and promoting engulfment by phagocytes

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Cell Mediated Immunity
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T-Lymphocytes

• After activation the cell divides to form
• T-helper cells secrete CYTOKINES
• ? help B cells divide
• ? stimulate macrophages
• Cytotoxic T cells (killer T cells)
• ? Kill body cells displaying antigen
• Memory T cells
• ? remain in body

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How T-cells work
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FUNCTIONING OF THE IMMUNE SYSTEM
HUMORAL (ANTIBODY MEDIATED) IMMUNE RESPONSE
CELL MEDIATED IMMUNE RESPONSE
ANTIGEN (1ST EXPOSURE)
ENGULFED BY
MACROPHAGE
ANTIGENS DISPLAYED BY INFECTED CELLS ACTIVATE
FREE ANTIGENS DIRECTLY ACTIVATE
BECOMES
APC
STIMULATES
HELPER T CELLS
CYTOTOXIC T CELL
B CELLS
STIMULATES
STIMULATES
MEMORY HELPER T CELLS
GIVES RISE TO
GIVES RISE TO
STIMULATES
STIMULATES
STIMULATES
ANTIGEN (2nd EXPOSURE)
ACTIVE CYTOTOXIC T CELL
MEMORY B CELLS
PLASMA CELLS
MEMORY T CELLS
STIMULATES
SECRETE ANTIBODIES
Defend against intracellular pathogens and cancer
by binding and lysing the infected cells or
cancer cells
Defend against extracellular pathogens by binding
to antigens and making them easier targets for
phagocytes and complement
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Role of antigen receptors in the immune response

• Both B cells and T cells carry customized
  receptor molecules that allow them to recognize
  and respond to their specific targets.
• The B cells antigen-specific receptor that sits
  on its outer surface is also a sample of the
  antibody it is prepared to manufacture this
  antibody-receptor recognizes antigen in its
  natural state.
• The T cells receptor systems are more complex. T
  cells can recognize an antigen only after the
  antigen is processed and presented in combination
  with a special type of major histocompatibility
  complex (MHC) marker.
• Killer T cells only recognize antigens in the
  grasp of Class I MHC markers, while helper T
  cells only recognize antigens in the grasp of
  Class II MHC markers. This complicated
  arrangement assures that T cells act only on
  precise targets and at close range.

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Role of cytokines in immune response

• Cytokines are diverse and potent chemical
  messengers secreted by the cells of your immune
  system. They are the chief communication signals
  of your T cells. Cytokines include interleukins,
  growth factors, and interferons.
• Lymphocytes, including both T cells and B cells,
  secrete cytokines. Cytokines are also secreted by
  monocytes and macrophages. Interferons are
  naturally occurring cytokines that may boost the
  immune systems ability to recognize cancer as a
  foreign invader.
• Binding to specific receptors on target cells,
  cytokines recruit many other cells and substances
  to the field of action. Cytokines encourage cell
  growth, promote cell activation, direct cellular
  traffic, and destroy target cells--including
  cancer cells.
• When cytokines attract specific cell types to an
  area, they are called chemokines. These are
  released at the site of injury or infection and
  call other immune cells to the region to help
  repair damage and defend against infection.

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Immunity to Infection

• Immunity is the acquired ability to defend
  against infection by disease-causing organisms.
• The adaptive immune system is responsible for
  immunity.

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Vaccines

• The word vaccination comes from vacca, which is
  Latin for cow.
• Edward Jenner could be considered the father of
  vaccination as he developed a method of
  protecting people from smallpox.
• He noticed that milkmaids who had previously been
  infected with cowpox (similar disease but milder)
  did not catch smallpox.
• In 1796, Jenner deliberately infected a small boy
  with material from a cowpox pustule, then six
  weeks later infected the boy with material from a
  smallpox pustule. The boy survived!
• Our current understanding of pathogens indicates
  that Jenner got lucky not all dangerous
  diseases have a less pathogenic equivalent as was
  the case with smallpox and cowpox.

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Types of Vaccine

• There are four main types of vaccinations
• Live attenuated vaccines
• Killed vaccines
• Toxoid vaccines
• Component vaccines
• Many vaccines contain adjuvants. This is a
  general term given to any substance that when
  mixed with an injected immunogen will increase
  the immune response. Examples of adjuvants
  include aluminium hydroxide and aluminium
  phosphate.

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Live attenuated vaccines

• Contain bacteria or viruses that have been
  altered so they can't cause disease.
• Usually created from the naturally occurring germ
  itself. The germs used in these vaccines still
  can infect people, but they rarely cause serious
  disease.
• Viruses are weakened (or attenuated) by growing
  them over and over again in a laboratory under
  nourishing conditions called cell culture. The
  process of growing a virus repeatedly-also known
  as passing--serves to lessen the disease-causing
  ability of the virus. Vaccines are made from
  viruses whose disease-causing ability has
  deteriorated from multiple passages.
• Examples of live attenuated vaccines include
• Measles vaccine (as found in the MMR vaccine)
• Mumps vaccine (MMR vaccine)
• Rubella (German measles) vaccine ( MMR vaccine)
• Oral polio vaccine (OPV)
• Varicella (chickenpox) vaccine

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Killed vaccines

• Contain killed bacteria or inactivated viruses.
• Inactivated (killed) vaccines cannot cause an
  infection, but they still can stimulate a
  protective immune response. Viruses are
  inactivated with chemicals such as formaldehyde.
• Examples of inactivated (killed) vaccines
• Inactivated polio vaccine (IPV), which is the
  injected form of the polio vaccine
• Inactivated influenza vaccine

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Toxoid vaccines

• Contain toxins (or poisons) produced by the germ
  that have been made harmless.
• Toxoid vaccines are made by treating toxins (or
  poisons) produced by germs with heat or
  chemicals, such as formalin, to destroy their
  ability to cause illness. Even though toxoids do
  not cause disease, they stimulate the body to
  produce protective immunity just like the germs'
  natural toxins.
• Examples of toxoid vaccines
• Diphtheria toxoid vaccine (may be given alone or
  as one of the components in the DTP, DTaP, or dT
  vaccines)
• Tetanus toxoid vaccine (may be given alone or as
  part of DTP, DTaP, or dT)

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Component vaccines

• Contain parts of the whole bacteria or viruses.
• These vaccines cannot cause disease as they
  contain only parts of the viruses or bacteria,
  but they can stimulate the body to produce an
  immune response that protects against infection
  with the whole germ.
• Component vaccines have become more common with
  the advent of gene technology, as the antigenic
  proteins can be identified and cloned then
  expressed in a laboratory to provide material for
  vaccination.
• Examples of component vaccines
• Haemophilus influenzae type b (Hib) vaccine
• Hepatitis B (Hep B) vaccine
• Hepatitis A (Hep A) vaccine
• Pneumoccocal conjugate vaccine

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How do diseases evade the immune response?

• Pathogens that infect the human body have evolved
  a number of different techniques for avoiding the
  immune response.
• These include
• Antigenic variation
• Antigenic mimicry
• Evading macrophage digestion
• Hiding in cells
• Immune suppression
• Disarming antibodies

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Avoiding the immune response

• Antigenic variation
• Some species of protozoan parasites evade immune
  response by shedding their antigens upon entering
  the host.
• Others (e.g. trypanosomes and malarial parasites)
  can change the surface antigens that they express
  so that the specific immune system needs to make
  a new antibody to respond to the infection. This
  is known as antigenic variation.
• Antigenic mimicry
• This involves alteration of the pathogens
  surface so that the immune system does not
  recognise the pathogen as non-self.
• Blood flukes can hijack blood group antigens from
  host red blood cells and incorporate them onto
  their outer surface so that the immune system
  does not respond to the infection.

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Avoiding the immune response

• Evading macrophage digestion
• Macrophages have an important role in the immune
  system as they phagocytosis and destroy foreign
  material. Some microbes (e.g. Leishmania) are
  able to avoid enzymatic breakdown by lysosomes
  and can remain and grow inside the macrophage
  this means they are able to avoid the immune
  system.
• Some bacteria can avoid phagocytosis by releasing
  an enzyme that destroys the component of
  complement that attracts phagocytes.
• Other bacteria can kill phagocytes by releasing a
  membrane-damaging toxin
• Hiding in cells
• Bacteria such as heliobacter can invade the
  epithelial lining of the intestine to multiply
  and divide, then transfer into neighbouring cells
  without entering the extracellular space where
  they would be vulnerable to detection.

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Avoiding the immune response

• Immune suppression
• Most parasites are able to disrupt the immune
  system of their host to some extent.
• HIV is an example of this. It selectively
  destroys T helper cells, therefore disabling the
  host immune system.
• Disarming antibodies
• Bacteria such as Staphylococcus aureus have
  receptors on their surface that disrupt the
  normal function of the hosts antibodies.
• These receptors bind to the constant region (the
  stem) rather than the normal antigen binding
  sites. This prevents normal signalling between
  antibodies and other parts of the immune system
  such as complement activation or initiating
  phagocytosis of a bound antigen.

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Invader antigens are everywhere!
What does it need to get by?
Skin!
neutrophils
Monoctyes (macrophages)
Invader dies!
T - Helper lymphs
B lymphs
More T - Helper lymphs!
Cytotoxic T lymphs
Invader dies!!
Plasma B cells
Memory B cells
Invader dies!!
Antibodies!!
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Immunity

• We have natural or innate resistance to certain
  illnesses including most diseases of other animal
  species.
• Immunity involves a specific defenseresponse by
  the host to invasion byforeign organisms or
  substances
• Acquired immunity is the protectionthat develops
  against specific microbes or foreign substances.

• Active immunity develops after exposure to
  microorganisms or foreign substances
• Passive immunity is acquired when antibodies are
  transferred from one person to another.

89
Naturally Acquired Immunity
Active
Antigens enter the body naturally, as when
Microbes cause the person to catch the disease.
There is a sub-clinical infection(one that produces no evident symptoms).
The body produces specialized lymphocytes and antibodies.

Passive
Antibodies pass from the mother to the fetus via the placenta during pregnancy or to her infant through her milk. The infant's body does not produce any antibodies of its own.


90
Artificially Acquired Immunity
Active
Antigens (weakened or dead microbes or their fragments) are introduced in vaccines.
The body produces specialized lymphocytes and antibodies.

Passive
Preformed antibodies in an immune serum are introduced into the body by injection(e.g. anti-venom used totreat snake bites). The body does not produceany antibodies
.

91
Induced Immunity
92
Active and Passive Immunity

• Active immunity
• Lymphocytes are activated by antigens on the
  surface of pathogens
• Natural active immunity - acquired due to
  infection
• Artificial active immunity vaccination
• Takes time for enough B and T cells to be
  produced to mount an effective response.

93
Active and Passive Immunity

• Passive immunity
• B and T cells are not activated and plasma cells
  have not produced antibodies.
• The antigen doesnt have to be encountered for
  the body to make the antibodies.
• Antibodies appear immediately in blood but
  protection is only temporary.

94
Active and Passive Immunity

• Artificial passive immunity
• Used when a very rapid immune response is needed
  e.g. after infection with tetanus.
• Human antibodies are injected. In the case of
  tetanus these are antitoxin antibodies.
• Antibodies come from blood donors who have
  recently had the tetanus vaccination.
• Only provides short term protection as abs
  destroyed by phagocytes in spleen and liver.

95
Active and Passive Immunity

• Natural passive immunity
• A mothers antibodies pass across the placenta to
  the foetus and remain for several months.
• Colostrum (the first breast milk) contains lots
  of IgA which remain on surface of the babys gut
  wall and pass into blood