The third line of defence protects the body against foreign infection. It is called into action when non-specific defences are not enough. The third line consists of specific immune responses mediated by antibodies, B cells, and T cells. When a pathogen invades, B cells and T cells work together to recognize and eliminate the pathogen. B cells produce antibodies targeted to specific antigens, while T cells destroy infected cells and regulate the immune response. Memory B and T cells provide long-term immunity against reinfection.

2. Presentation
Topics
IMMUNE SYSTEM

3. Presentation
Made By
Junaid Ahmed
BS Plant Sciences Quaid-i-Azam University
Islamabad

4. What is Immunity?
Ability of the body to remove intruders and
the toxics produced by intruders.
It is classified
Innate Immunity
Acquired Immunity

5. Innate immunity:
Also known as Native immunity
It is resistance to infections that an individual posse
sses by virtue of his
genetic and constituitional makeup
It may be :
Specific
Non specific

6. Acquired Immuniity:
The resistance that human acquires during life is known as
Acquired Immunity
Also known as Adaptive Immunity
It is again classified on the level of species,race
Species:entire human species is resistance to the plant
pathogens
Racial immunity:
Particular races of that species are resistant e.g negroid
species in USA is more prone to TB than causcian species.

7. First line of defence
The first and obviously best defence is first
line of defence is to keep microbes out in
the first place.
Human body has two surfaces exposed to the
environment:
Skin
Mucous membrane of the digestive system a
nd respir-atory tracts.

8. Structure and layers of skin
Skin is made up of two layers
Epidermis
Dermis
Glands of skin are
Sebaceous gland(located in the dermis)
it produce an oily substance sebum.
Sweat glands :produce perspiration
Perspiration: flush microorganisms form the surface of the skin.
Perspiration also contain
Salt
Antimicrobial
lysozyme

9. Skin-Epidermis
Consists of closely packed cells :keratinocytes
Keratins: resistant to bacterial enzymes and toxins.
Outer skin cells sheds continuously
Lower PH(3-5):discourage growth of many microbes.

11. Mucous membranes
Mucous membrane or mucosa is a membrane that
lines various cavities in the body and surrounds
internal organs (gastrointestinal tract and respiratory
tract)
Mucous membrane secretes mucus :that trap and
move microbes as a ciliary escalator.

12. Role of acids and enzymes
Tears and saliva contain lysozyme that can actually kill bacteria
.
Stomach contain hydrochloric acid secreted by oxyntic or
parietal cell.it kills the bacteria present in food.

13. Role of respiratory tract
The respiratory passage is lined with mucous.
Cilia also lines the respiratory passage.
There is mucous in the respiratory passage that traps invading
microbe and other foreign debris with the cilia
Urine flow flushes out flushes out pathogens from the bladder
area.
Friendly bacteria also protect us from harmful pathogens.
Neutrophils also play important for our defence.

14. Summarization of barriers against pathogens:

15. Second line of defence:
“The non specific internal
defence that is innate immunity
that combat all invading
microbes.’’

16. When it is activated:
If the pathogens are able to get past the first line of
defence for example through cut in our skin ,and an
infection develops, the second line of defence
becomes active.
Second line of defence is a group of cells, tissues
and organs that work together to protect the body

18. Killing cells of blood:
The cells involved are leukocytes ,which seek out and
destroy disease causing organisms .
Neutrophils
T helper cells
cytotoxic T cells (killing cells)
Macrophages
Dendritic cells
B cells
Supressor T cells

19. INFLAMMATORY RESPONSE
The inflammatory response is the major component
of
non-specific defence.
Any damage to tissue, whether caused by an
infectious microorganism or by physical inJury, even
just a scratch of insect bite triggers this response.

20. Continue
The change of events that make up the inflammatory response, in
case where a pin has broken the skin and infected it with bacteria
The first thing that happened when a tissue is injured is that the
damaged cells release chemical alarm signals such as histamine.
The chemical sparks the mobilization of various defences. Histamine
for instance induces neighboring blood vessels to dilate and blood
vessels start licking.
Blood flood to damaged area increases and blood plasma passes out
of the leaky vessels into the interstitial fluid of affected tissue the
major result of inflammatory response is to disinfect and clean
injured tissue.

21. Continue
The white blood sells engulf bacteria and the remains of body cells
are killed by them.
The pus that collect around a wound consist of microbes, living and
dead white blood cells. the inflammatory helps to prevent the spread
of infection.
The body may react with one more several inflammatory weapons for
instance the number no white blood cells circulating in the blood
may increase.
Another response is fever

22. FEVER
Fever is nonspecific protection against infection. The invader or
the responder to an invader causes a part of brain called
hypothalamus to increase the body temperature. Fever may
increase body metabolism, speeding up the repair process. Fever
may also slow down the reproduction of some bacteria and
viruses

23. When a pathogen (disease-causing organism) invades
the body, the neutrophils gather at the entry site and
try to engulf it and destroy it. Should the invaders get
past the neutrophils, several things may happen. The
macrophages (big eaters) will be attracted by the death
throes of the neutrophils. These cells attempt to engulf
the invader, but they also send signals to other cells
for help.
HOW IT WORKS

24. dendritic cells, which are constantly scouting
around (they can communicate with up to 200
other cells at once) may find the pathogen and
take a piece to present to T helper cells
that congregate in the lymph nodes in the
body.

25. A T helper cell will recognize only one pathogen
. If the T helper cell recognizes the pathogen it
activates the appropriate immune cells for the att
ack.

26. T helper cell response to virus infection
If the invading organism is a virus, the T helpe
r cell will signal the cytotoxic (killer) T cells to
come to the rescue. These cells punch holes in
the walls of invading virus-infected host cells,
killing the cells and destroying the virus. B cells
can also be activated to produce antibodies that
will stick to free viruses, marking them out for
macrophages to clean up.

28. If the invader is bacterial, B cells are usually
drawn to the task. They squirt an antibody (IgG) on to the
bacteria (a bit like squirting tomato
sauce onto chips). This sticks everything together and
makes it attractive to macrophages, which
clear up the mess. If the bacteria enters the system
through the gut, the antibodies released by the B cells are
usually IgA antibodies. These antibodies acts by blocking
the
bacteria adhering to the
gut cells, preventing infection.

29. After a few days of fighting off the pathogen, we
will recover from our infection. At this
stage, the immune system activates T suppressor
cells. These cells send out ‘calm down’
signals, which quieten down the immune
system.

30. • After a few days of fighting off the pathogen,
we will recover from our infection. At this stag
e, the immune system activates T suppressor c
ells. These cells send out ‘calm down’ signals,
which quieten down the immune system.

31. The ThirdLine of Defence:
“Protecting the body against foreign infection”
•It is called into Action when non specific methods are not enough and
infection becomes widespread.
•It distinguishes between self and non self and only reacts against non
self (foreign).
•Third line of defense consists of a specific immune response and is the
bodies main immune response.
•It helps the body to build up a resistance and fight long term infections.
•Components of third Line of Defence:
Antibodies ,T cells and B cells .

32. Historically division of Third Line of Defence:
• It is divided into TWO types:
•Humoral Immunity:
It is called as “Humoral” because it involves HUMORS (fluid).
“The immune response involving the transformation of B cells into plasma cells that
produce and secrete antibodies to a specific antigen “
Because Antibodies are involved now it is called as Antibody mediated immunity.
•Cell mediated Immunity:
Type of Immunity which does not involve Antibodies ,rather involves the activation of
phagocytes,T lymphocytes, and the release of various Cytokines in response to CD4
cells or Helper T cells.

33. ThirdLine Of Defence:
Each Antibody is specific to the antigen that has invaded the body .For
example: the antibody to fight strep throat will not fight a cold virus .

34. Components of Thirdline of defense:
Antibody :
•Antibodies (also called immunoglobulins or Ig's) are Y-shaped
proteins that circulate through the blood stream and bind to
specific antigens, thereby attacking microbes
•The plasma cells are highly developed and are able to make
several thousand antibody molecules every second.
•Only the protein (antibody) molecules that they make move
around the body. These proteins are released into the blood
and carried to the area of infection

35. Antigen Antibody Complex:
The antibody molecule, on the other hand, binds to the antigen in a similar way to a substrate
binding with an enzyme. The fit, however is not as precise as the enzyme-substrate complex.

36. Antibodies can react in a number of ways:
• They can combine with the antigen to cover it’s active site.
• It may dissolve parts of the cell wall.
• It can neutralize the toxins produced by the antigen.
• IgA causes the bacteria to clump together and become less
active so that it can be taken more easily to the lymph.
• It can make the antigen more susceptible to phagocytosis.

37. Antibody production via the Activation of Helper T cells:

38. Continued:
T cells:
•T cells (or T lymphocytes) are types of non-phagocytotic white blood cells that produce a ‘cell
mediated response’ and destroy antigens.
•They derive from bone marrow stem cells that migrate to the thymus to become T cells. They
circulate through the blood, lymph and spleen and are inactive until they come into contact with an
antigen.
•The antigen combines with the antibody and activates it to make copies of itself.
•Once activated, some T cells secrete substances that can directly destroy antigens, others cause the
bacteria to clump together so they can be taken more easily to the lymph, while others stimulate B cell
s and macrophages.
•After an infection, some T cells remain in the body as memory cells ready incase a similar infection
•of the same pathogen occurs.

39. Types of T cells:
•Killer T Cells, or Cytotoxic T cells (Tc) secrete chemicals that destroy antigens directly or
inhibit replication of viruses. They also enhance macrophage activity. Cytokines help co-ordinate
B & T cells. Natural killer cells are a type of cytotoxic T cell that destroys abnormal host cells
such as cancer cells or those infected by viruses.
•Helper T Cells (Th) secrete interleukins (hormone like messenger chemicals between l
ympocytes) which enhance antibody production by B cells and helps regulate cytotoxic T cells . H
IV infects T4 helper cells which reduces their number and function. They can also stimulate ma
crophages for phagocytosis.

40. Continued:
•Memory T Cells retain the ability to recognise the original invading antigen. Future infections can
be dealt with much quicker and more effectively as they can produce large numbers of antibodies very
quickly.
•Suppressor T Cells help turn off the immune response once the antigen is destroyed and the i
nfection is under control.

41. Continued:
B Cells:
•B cells are non-phagocytotic white blood cells and provide the ‘antibody mediated’ response.
•They derive from bone marrow stem cells that stay and develop in the bone marrow and later
accumulate in the spleen and lymph nodes.
•The presence of antigens stimulates the lymphocytes to divide by mitosis to clone themselves.
They then differentiate into plasma cells that send antibodies into the blood, or into memory cells.
This usually occurs in the lymph node and causes it to swell when the body is fighting an infection.
•The plasma cells secrete antibodies with shape that is compatible with the antigen order to form the
antibody-antigen complex.

42. Types of B cells:
Memory B cell : long lived provide future immunity
Plasma Cells: secrete antibodies into blood stream.

43. B and T cell interaction:
•Once an antigen enters the body it is ingested and processed by a macrophage. When engulfing an
antigen, macrophages display fragments of the antigen on their outer membrane which B & T cells
recognize.
•B & T cells can collaborate because they are in close proximity and often make contact with each o
ther.
•They are also both regulated by cytokines. Cytokines are proteins secreted by T cells and m
acrophages that signal to other cells to initiate the immune response (such as B cells transforming to
plasma cells).
•B cells cannot function without T cells that stimulate them to produce antibodies, and helper T cells
induce B cells to divide and produce large numbers of clones.

44. Cont.

45. Types Of Immunity

46. There are two basic types of immunity
•Inborn or Innate immunity
•Acquired immunity
There are two components of the acquired
immune system.

47. Types of acquired immunity
There are two ways to acquire adaptive immunity
Active immunity
Passive immunity

48. Active immunity
Active immunity involve the production of
antibodies by the body itself and the subsequent
development of memory cell .
Examples of active immunity
Natural active immunity
Artificial active immunity

49. Natural active immunity:
Producing antibodies in response to exposure to a
pathogenic infection .
Artificial active immunity:
Producing antibodies in response to control
exposure to an attenuated pathogen

50. Passive immunity
Passive immunity results from acquisition of
antibodies from another source and hence
memory cells are not develop.
Examples of passive immunity
natural passive immunity
artificial passive immunity

51. Natural passive immunity:
Producing antibodies from another organism
Artificial passive immunity:
Receiving manufacturing antibodies by an
external delivery.

52. Immune response
The immune system protect the body from
possibly harmful substances by recognizing and
responding to antigens.
Antigens are substances on the surface of cells,
viruses , fungi or bacteria
Non living substances such as toxins, chemicals ,
drugs and foreign particles can also be antigens

53. Antibody MEDIATED Immunity
(Humoral Immune response)

54. Humoral Immunity
Humoral immunity is the aspect of
immunity that is mediated by macromolecules
found in extracellular fluids such as secreted
antibodies, complement proteins, and certain
antimicrobial peptides. Its aspects involving
antibodies are often called antibody-mediated
immunity.

55. Humoral Immune Response
The humoral immune response involving the
transformation of B cells into plasma cells that
produce and secrete antibodies to a specific antigen.
In the humoral immune response antibodies help
neutralize or eliminate toxins and pathogens in the blood
and lymph

58. B Cells
Lymphocytes that react directly
with antigens
Require stimulation from helper T
Cells
Offspring become
1.Plasma Cells
2. Memory Cells
Humoral Immunity is the
immunity that
 provided by Antibodies

59. Antigin Recognition by B Cells & Antibody
Each B cell antigen receptor is a Y-shaped molecule with two identical
heavy chains and two identical light chains
The constant regions of the chains vary little among B cells, whereas the
variable regions differ greatly
The variable regions provide antigen specificity
Binding of a B cell antigen receptor to an antigen is an early
step in B cell activation
This gives rise to cells that secrete a soluble form of the protein called an
antibody or immunoglobulin (Ig) Secreted antibodies are similar to B
cell receptors but lack transmembrane regions that anchor receptors in
the plasma membrane

63. Antigin Recognition by T Cells
Each T cell receptor consists of two different polypeptide chains
(called α and β)
The tips of the chain form a variable (V) region;the rest is a constant
(C) region
T cell and B cell antigen receptors are functionally different T cells
bind to antigen fragments displayed or presented on a host cell
These antigen fragments are bound to cell surface proteins called
HC molecules
MHC (major histocompatibility complex) molecules are host
proteins that display the antigen fragments on the cell surface

65. In infected cells, MHC molecules bind and transport antigen
fragments to the cell surface, a process called antigen
presentation
A T cell can then bind both the antigen fragment and the MHC
molecule
This interaction is necessary for the T cell to participate in the
adaptive immune response

67. Mature B cells circulate in the blood and lymph and are
carried to the secondary lymphoid organs e.g. lymph
nodes and spleen
Antigens are also carried by blood or lymph to the
secondary lymphoid organs
B cells usually meet their specific antigens in these organs
An antigen binds to a B cell (or a clone of B cells) which
carries mIgM specific for that antigen and activates it
(clonal selection)
B cell activated by an antigen starts to proliferate and
increase number of cells (clonal expansion)
B cell activation

68. Antibody production
Plasma cells secrete antibodies
Secreted antibodies have the same specificity as the mIgM on the
surface of B cell from which the plasma cell is derived.
Antibodies are the effector molecules of humoral immune response
that bin with the antigen and eliminate the microbe
Initially plasma cells secrete IgM antibody
Within a few days antibody class switching occurs and they secrete IgG
(or IgA or IgE)
Memory B cells have a long life span and provide heightened immune
response if the person encounters the same antigen in future.

69. Antibody Function
Antibodies do not kill pathogens; instead they mark pathogens for
destruction
In neutralization, antibodies bind to viral surface proteins preventing
infection of a host cell
Antibodies may also bind to toxins in body fluids and prevent them
from entering body cells.
In opsonization, antibodies bind to antigens on bacteria creating a
target for macrophages or neutrophils, triggering phagocytosis
Antigen-antibody complexes may bind to a complement protein which
triggers a cascade of complement protein activation
Ultimately a membrane attack complex forms a pore in the membrane
of the foreign cell, leading to its lyses.

71. Antibody Molecule
Consists of 4 peptide chains
• 2 identical heavy (H) chains (MW 50 kD)
• 2 identical light (L) chains (MW 25 kD)
 Each L chain is bound to a H chain by disulfide bond and form
H-L combinations
H-L combinations are held together by disulfide bond between
H chains
Amino acid sequence in amino terminal is variable ( V region) and
form the antigen binding site (hypervariable region or CDR)
Amino acid sequence in carboxylterminal is relatively constant
(C region or Fc) and is responsible for biological function e.g.
complement fixation, placental transfer, opsonization etc.

72. Antibody Structure

73. Life Span of Antibody
Antibodies, as noted, are protein molecules. They will either leave the
body in secretions or be broken down by enzymes that degrade protei
ns in the body. The average life span of most antibody molecules is ro
ughly a few weeks. Continued antigen stimulation is necessary to con
tinue the antibody response, and as the antigen disappears, the stimula
tion disappears. The antibodies produced as a result of the initial stim
ulation remain in the bloodstream for a long period of time, with abou
t half the quantity disappearing after a few weeks and half the remaini
ng disappearing after another few weeks, and so forth. In many cases,
the antibody level is sufficient to sustain an individual for the remaind
er of his or her life, especially after recovery from disease.

74. Vaccination and Antibodies
Vaccines prepare your immune system to fi
ght disease by taking advantage of the fact
that the immune system can ‘remember’ in
fectious organisms. Vaccination gives us i
mmunity without us having to experience t
he disease or its symptoms.

75. Immunological Memory
Immunological memory is responsible for longterm protections against
diseases, due to either a prior infection or vaccination
The first exposure to a specific antigen represents the primary immune
response
During this time, selected B and T cells give rise to their effector forms
In the secondary immune response, memory cells facilitate a faster, more
efficient response

76. Specific defense mechanisms

77. Cell mediated immune response
Consists of t-cells.
Originate in the bone marrow.
T lymphocytes become mature in thymus gland. So
they are called t lymphocytes.
They recognize antigen and combat with them.
The antigen may be any foreign object like microor
ganisms within the host cell, foreign tissue in case o
f organ transplant.

78. Kinds of T cells
There are three kinds of t-cells.
1. Cytotoxic T cells:- directly kill invaders.
2. Helper T cells:- aid B cells and other cells to do their job.
3. Suppressor T cells:- suppressor T cells suppress the activit
ies of B and other T cells so they don’t overreact.
The response in which lymphocyte directly kill the microorga
nisms by
phagocytosis is called cell mediated response.
Cells of immune system kill those cells of the body that have
been infected
with microganisms or that are cancerous.
This response relies on cytotoxic t cells.

79. The response in which lymphocyte directly kill the microorganisms
by
phagocytosis is called cell mediated response.
Cells of immune system kill those cells of the body that have been in
fected
with microganisms or that are cancerous.
This response relies on cytotoxic t cells.
The cellular immune response occur in two phases.
Activation phase:- Tc cells are activated and triggered to divide repe
atedly.
Effector phase:- Tc cells encounter target cells and kill them

81. Primary immune response
The primary immune response occurs when an
antigen comes in contact to the immune system for the
first time.
The immune system learn to recognize antigen and
how to make antibody against it.
And eventually produce memory lymphocytes.

82. Secondary immune response
The secondary immune response occurs when the second
time the person is exposed to the same antigen.
At this time point immunological memory has been
established and the immune system can start making
antibodies immediately.

83. Graph

84. Differences between Primary and Secondary immune response.
Primary immune response
1. This occur as a result of primary contac
t with an antigen.
2. Responding cell is native B cell and T c
ell.
3. Lag phase is longer.
4. Level of antibody reaches peak in 7 to 1
0 days.
Secondary immune response
1. This occurs as a result of second an
d subsequent exposure of the same a
ntigen.
2. Responding cell is memory cell.
3. Lag phase is shorter.
4. Level of antibody reaches peak in 3
to 5 days.

85. 5. It takes longer time to establish immunity
6. First antibody produced is mainly IgM. Althou
gh small amount of IgG are also produced.
7. Amount of antibody produced depends on nat
ure of antigen. Produce in low amount.
8. Antibody level declines rapidly.
9. Affinity of antibody is lower for its antigen.
10. Primary response appears mainly in the lymph
nodes and spleen.
5. It takes shorter time to establish immunity.
6. Mainly IgG antibody is produced . Although som
etimes small amount of IgM are produced.
7. Usually 100-1000 times more antibodies are prod
uced
8. Antibody level remain high for longer period.
9. Antibodies have greater affinty for its antigen.
10. Secondary immune response appers mainly in the
bone marrow, followed by the spleen and lymph
nodes.
Primary immune response Primary immune resonce

86. Evolution of Immunity in
Vertebrates

87. Invertebrates
Vertebrates
Evolutionary development

88. First Observation about Immunity
Élie Metchnikoff
|
Larvas of Starfish in bottle
|
|
Placed a rose Thorn
|
|
Thorn Covered With the
Larval Cells
|
|
Attempting to engulf
|
Phagocytosis

89. Phagocytes
All animals have phagocytic cells that attack invading
microbes.
These cells travel through the animal’s circulatory system.
In animals that do not have a circulatory system the phago
cytic cells roam inside the mesoglea.

90. Distinguishing Self from Nonself
The ability to recognize the difference between cells of your own
body and cells from another individual seems to have evolved
early in the history of life.
Sponges, thought to be the most ancient animals, attack tissues
from other sponges just like insects and starfish.
None of these invertebrates seem to show any evidence of having
an immune system.
Apparently the humoral immune defense did not evolve until
vertebrates.

91. Lymphocytes
Lymphocyte – A type of white blood cell responsible for the
immune responses, two classes; B cells and T cells.
Invertebrates do not have complementary proteins or lymp
hocytes.
But annelid earthworms and other invertebrates do have ly
mphocyte-like cells that may be evolutionary precursors of
lymphocytes.

92. Invertebrates
In 1882 Elie Metchnikoff became the first to reco
gnize that invertebrate animals have immune def
enses.
The invertebrate immune response shares severa
l elements with the vertebrate one.

93. Innate Immunity of Invertebrates
In insects, an exoskeleton made of chitin
forms the
first barrier to pathogens
• The digestive system is protected by a
chitinbased barrier and lysozyme, an
enzyme that
breaks down bacterial cell walls
• Hemocytes circulate within hemolymph
and carry
out phagocytosis, the ingestion and diges
tion of
foreign substances including bacteria

94. Hemocytes also secrete antimicrobial peptides that disrupt the
plasma membranes of fungi and bacteria
The immune system recognizes bacteria and fungi by structure
s on their cell walls
• An immune response varies with the class of pathogen
encountered

95. Vertebrates
The earliest vertebrates, the jawless lampreys first evolved some
500 million years ago.
As invertebrtes was consist on innate immunity but the
development occured here that vertebrates consist s on both both
Innate and Adoptive Immunty with the verity of immune
soldiers
Such As Skin, Mucus Membranes , Blood etc
At this early stage of vertebrate evolution, however, lampreys
lack distinct populations of B and T cells that are found in all
higher vertebrates.

96. Innate Immunity of Vertebrates
• The immune system of mammals is the best
understood of the vertebrates
• Innate defenses include barrier defenses,
phagocytosis, antimicrobial peptides
• Additional defenses are unique to vertebrates:
natural killer cells, interferons, and the
inflammatory response

97. Barrier Defenses
Barrier defenses include the skin and mucous membranes
of the respiratory, urinary, and reproductive tracts
• Mucus traps and allows for the removal of
microbes
• Many body fluids including saliva, mucus, and tears are
hostile to many microbes
• The low pH of skin and the digestive system prevents
growth of many bacteria

98. Development to words Adaptive Immunity
Adaptive Immunity
|
Specific Immune Response
|
Self or Non self Recognition
|
Memory

99. The immune System & The Lymph System
Lymph organs include the bone marrow, lymph nodes, spleen, and thy
mus. Bone marrow contains tissues that produce lymphocytes.
B-cells mature in the bone marrow.
T-cells mature in the thymus gland.
Other blood cells like monocytes and leukocytes are made in the bon
e marrow.
The thymus makes the hormone; thymosin, that causes T-cells to matu
re in the thymus.