2. Innate immunity or nonspecific,
immunity is the natural resistance with
which a person is born. It provides
resistance through several physical,
chemical, and cellular approaches.
Microbes first encounter the epithelial
layers, physical barriers that line our skin
and mucous membranes.
2
3. Immunity
Immunity is defined as the state of resistance or in
susceptibility exhibited by the host to toxic
molecules,micro organisms and foreign cells.
Types of immunity: two types of immunity
1. Non-specific immunity or innate or natural immunity
2. Specific, acquired or adaptive immunity.
3
4. Types of immunity:
1.Species immunity
2.Racial immunity
3.Individual immunity
Host factors in innate immunity
a.Age
b.Hormonal influence
c.Nutrition
4
5. Species immunity
• Resistance to infection varies with the species
of animals.
• Animals of same species exhibited uniform
pattern of susceptibility to infections.
• For eg;disease of mammals do not effect fish
or reptiles and vice versa,Birds are immune to
tetanus.
5
6. Racial immunity
• Various animal and plant races show marked
differences in their resistance to certain
infectious disease.
• For eg:Algerian race of sheep are immune to
anthrax eventhough which is more common
disease among other races of sheep;Blacks
show high resistance to yellow fever and
malaria than white.
6
7. Individual immunity
• Factors affecting in individual immunity
• AGE:is an important indicator ,foetus and old
person are more susceptible to any diseases.In
foetus immune cells are immature and in old age
there is gradual wanning of immune cells.
• Hormonal influence:Hormonal imbalance are also
responsible for the susceptibility of an individual
to any disease.For eg;diabetes,hypothyroidism
and adrenal dysfunction influence the individuals
susceptibility to infection.
7
8. • Nutrition:Immune response is suppressed in
protein malnutrition and is deficiency of some
essential amino acids.Besides this other
factors such as personal hygiene,the nature of
work place and its hazards,the oppurtunity for
contacts with infected individual can also ply
an imp role in innate immunity.
8
9. Adaptive immunity is often sub-divided into two major types
depending on how the immunity was introduced. Naturally
acquired immunity occurs through contact with a disease
causing agent, when the contact was not deliberate, whereas
artificially acquired immunity develops only through
deliberate actions such as vaccination. Both naturally and
artificially acquired immunity can be further subdivided
depending on whether immunity is induced in the host or
passively transferred from a immune host. Passive immunity
is acquired through transfer of antibodies or activated T-cells
from an immune host, and is short lived -- usually lasting only
a few months -- whereas active immunity is induced in the
host itself by antigen, and lasts much longer, sometimes life-
long. The diagram below summarizes these divisions of
immunity.
9
11. Adaptive and innate immunity
• The immune system is typically divided into two
categories--innate and adaptive--although these
distinctions are not mutually exclusive.
• Innate immunity
– Innate immunity refers to nonspecific defense
mechanisms that come into play immediately or
within hours of an antigen's appearance in the body
– These mechanisms include physical barriers such as
skin, chemicals in the blood, and immune system cells
that attack foreign cells in the body
– The innate immune response is activated by chemical
properties of the antigen
11
12. Adaptive and innate immunity
• Adaptive immunity
– Adaptive immunity refers to antigen-specific immune
response
– The adaptive immune response is more complex than
the innate
– The antigen first must be processed and recognized.
Once an antigen has been recognized, the adaptive
immune system creates an army of immune cells
specifically designed to attack that antigen
– Adaptive immunity also includes a "memory" that
makes future responses against a specific antigen
more efficient.
12
13. Types of Acquired Immunity
I. Naturally Acquired Immunity: Obtained in the course
of daily life.
A. Naturally Acquired Active Immunity:
– Antigens or pathogens enter body naturally.
– Body generates an immune response to antigens.
– Immunity may be lifelong (chickenpox or mumps)
or temporary (influenza or intestinal infections).
B. Naturally Acquired Passive Immunity:
– Antibodies pass from mother to fetus via placenta
or breast feeding (colostrum).
– No immune response to antigens.
– Immunity is usually short-lived (weeks to months).
– Protection until child’s immune system develops.
13
14. Types of Acquired Immunity (Continued)
II. Artificially Acquired Immunity: Obtained by
receiving a vaccine or immune serum.
1. Artificially Acquired Active Immunity:
– Antigens are introduced in vaccines
(immunization).
– Body generates an immune response to antigens.
– Immunity can be lifelong (oral polio vaccine) or
temporary (tetanus toxoid).
2. Artificially Acquired Passive Immunity:
– Preformed antibodies (antiserum) are introduced
into body by injection.
• Snake antivenom injection from horses or
rabbits.
– Immunity is short lived (half life three weeks).
– Host immune system does not respond to antigens.
14
15. Adaptive and innate immunity
Components of the immune system
Innate immune system Adaptive immune system
Response is non-specific
Pathogen and antigen specific
response
Exposure leads to immediate
maximal response
Lag time between exposure
and maximal response
Cell-mediated and humoral
components
Cell-mediated and humoral
components
No immunological memory
Exposure leads to
immunological memory
Found in nearly all forms of life
Found only in jawed
vertebrates
15
18. Innate immunity
• This prevents entry of micro-organisms into tissues or, once they have
gained entry, eliminates them prior to the occurrence of disease
• Characteristics
– Present from birth.
– Non-specific - acts on many organisms and does not show specificity.
– Does not become more efficient on subsequent exposure to same
organisms.
• Prevention of entry of organisms
– Mechanical barriers at body surfaces, skin, mucous membranes -
disruption leads to infection.
– Antibacterial substances in secretions, lysozyme, lactoferrin, low pH
of stomach contents
– Prevention of stasis.
• Peristalsis/flow of urine/upward movement of secretions in
bronchial tree.
• Coughing
• Vomiting
18
20. Innate immunity
• Non-specific elimination of micro-organisms
1. Phagocytosis - ingestion and killing of micro-organisms
by specialised cells (phagocytes)
Phagocytes - polymorphonuclear leukocytes (neutrophils),
mononuclear phagocytes (monocytes, macrophages)
2. Opsonisation - the process of coating micro-organisms
with some of the proteins found in plasma, to make
them more easily phagocytosable
1. An OPSONIN is a plasma protein binding to bacteria. This
promotes adhesion between the opsonised bacteria and
macrophages because the opsonin binds to receptors on
phagocyte membrane e.g. complement with complement
receptors and phagocytes. Opsonisation and phagocytosis
are more efficient in immune individuals.
20
22. Adaptive immunity
• It has been observed that the immune system responds to
micro-organisms but not to its own cells and that the system
knows that the body has been infected previously with a
particular organism. This implies:
– Immunological recognition
– Self/non-self discrimination
– Immunological specificity
– Immunological memory
• Immunity is mediated by the IMMUNE SYSTEM, which
responds to infection by mounting an IMMUNE RESPONSE. An
immune response must:
– RECOGNISE a micro-organism as foreign (non-self) as distinct from self
– RESPOND to a micro-organism by production of specific antibodies
and specific lymphocytes
– MEDIATE the elimination of micro-organisms
– An agent which evokes an immune response is called
an IMMUNOGEN. The term ANTIGEN is applied to a substance which
reacts with antibody.
22
23. Key mediators of immunity
• A specialized group of cells termed as antigen-
presenting cells (APCs) link the innate and adaptive
immune systems by producing cytokines, which:
– Enhance innate immune cell function; and
– Contribute to lymphocyte function
• Phagocytes and lymphocytes are key mediators of
immunity
• Phagocytes are first line of defense against infection
• Lymphocytes have specialized function and mediate
adaptive immune response
23
24. Antigens
• Antigens are molecule recognized by receptors in
lymphocytes
• Originally the term antigen was used for any molecule
that induced B cells to produce a specific antibody
(antibody generator)
• This term now more widely used to indicate molecules
that are specifically recognized by antigen receptors of
either B cells or T cells
• So now broadly defined as molecules that initiate
adaptive immune responses (e.g. components of
pathogenic organisms); can also be called immunogen
• A large variety of self molecules can serve as antigen
molecule as well, provoking autoimmune responses that
can be highly damaging, and even lethal
24
25. Antigens
• Antigens are the initiators and driving forces of all
adaptive immune responses
• When antigen is eliminated, immune responses
switch off
• Both T cell receptors and immunoglobulin
molecules (antibody) bind to their respective
antigens with a high degree of specificity
• They have similar structural relationships and are
closely related in their evolution, but bind to very
different types of antigens and carry out different
biological functions
25
26. Antigens
Epitope:
Small part of an antigen that interacts with
an antibody.
Any given antigen may have several
epitopes.
Each epitope is recognized by a different
antibody.
26
28. Antibody
• Soluble antibodies (immunoglobulins) are a group of serum
molecules closely related to and derived from antigen
receptors and B cells
• Basic Y-shaped structure with two regions (variable region)
at tip of Y that binds with antigen
• Stem of Y is called constant region and is not involved in
antigen binding
• The two variable region contains identical antigen binding
sites that are specific to only one type of antigen
• However the amino acid sequence in variable region of
different antibodies are extremely variable, therefore
provide an extremely large stock of antigen-binding sites
28
30. Antibody
• Pathogens typically have many different antigens
on their surface
• Each antibody binds to an epitope, which is a
restricted part of an antigen
• A particular antigen can have several different
epitopes or repeated epitopes
• Antibodies are specific to epitopes rather than
the whole antigen molecule
• The constant region of antibody (Fc region) acts
as adapters to link phagocytes to pathogens
30
32. Immune responses
• There are two phases of immune response – antigen
recognition and antigen eradication
• Most immune responses to infectious organisms are
made up of a variety of innate and adaptive
components:
– In the earliest stages of infection, innate responses
predominate
– Later the lymphocytes start to generate adaptive immune
responses;
– After recovery, immunological memory remains within the
population of lymphocytes, which can then mount a more
effective and rapid responses if there is a reinfection from
same pathogen later
32
33. Antigen recognition
• Lymphocytes are responsible for antigen recognition, and
this is achieved by clonal selection
• Each lymphocyte is genetically programmed to be
capable of recognizing just one particular antigen but as
a whole immune system can recognize many thousands
of antigens collectively (collective proportion of specific
lymphocytes)
• When an antigen binds to few lymphocytes that can
recognize it , they are induced to proliferate rapidly
which results in sufficient number of lymphocytes to
mount an adequate immune response
• Clonal selection – antigen selects and activates the
specific clones to which it binds; this operates for both B
and T cells 33
35. Antigen recognition
• Lymphocytes that have been stimulated by binding to
their specific antigen, take the first step towards cell
division
– They express new receptors that allow them to respond to
cytokines from other cells, which signal proliferation
– May start to secrete cytokines themselves
– Will usually go through a number of cycles of division
before differentiating into mature cells under the influence
of cytokines
• Memory cells – even when infection has been
overcome, some of the newly produced lymphocytes
remain, available for restimulation if antigen is ever
encountered again; these cells are called memory cells
and can provide immunity to the infection later when
needed
35
36. Antigen eradication
• The defense mechanisms by which immune
system can destroy pathogens, each being
suited to given type of infection at a particular
stage of its lifecycle, are often referred to as
effector systems
1. Antibody binding
2. Phagocytosis
3. Cytotoxic reactions
36
37. Effector system
• Antibody binding
– One of the simplest effector systems
– Antibodies can combat certain pathogens just by
binding to them
– E.g. antibody to the outer coat proteins of some
rhinovirus (which causes colds) can prevent the
viral particles from binding to and infecting host
cells
37
38. Effector system
• Phagocytosis
– Antibodies activates complement or acts as an
opsonin to promote ingestion by phagocytes
– Phagocytes that have bound to an opsonized microbe
engulf it by extending pseudopodia around it
– These fuse and internalize microorganism
(endocytosed)in a phagosome
– Granules and lysosomes fuse with the phagosone,
pouring enzyme into resulting phagolysosome, to
digest the contents
38
40. Effector systems
• Cytotoxic reactions
– These are directed against whole cells which are too
large for phagocytosis
– The target cell may be recognized either by
• specific antibody bound to the cell surface
• T cells using their specific TCRs
– The attacking cells direct their granules towards the
target cell which are discharged into extracellular
space close to the target cells
– These granules contain perforin molecules which can
punch holes in outer membrane of the target
– Some cytotoxic cells can signal to the target cell to
initiate programmed cell death or cell suicide – a
process called apoptosis
40
42. Immune responses to extracellular and intracellular
pathogens
• In dealing with extracellular pathogens, the immune
system aims to destroy the pathogen itself and
neutralize its products
• In dealing with intracellular pathogens, the immune
system has two functions
– T cells can destroy the infected cells (i.e. cytotoxicity)
– T cells can activate the infected cells to deal with the
pathogen itself (e.g. helper T cells release cytokines, which
activate macrophages to destroy the organisms they have
internalized)
• Many pathogens have both intracellular and
extracellular phases of infection, so different
mechanisms are usually effective at different times
42
44. Immune responses to extracellular and intracellular
pathogens
• For example, the polio virus travels from the gut,
through the blood stream to infect nerve cells in the
spinal cord
• Antibodies and complement can block the extracellular
phase of the life cycle and promote phagocytosis of the
virus
• Interferons produced by infected cells signal uninfected
cells to induce state of antiviral resistance
• Virus can multiply only within living cells; CTLs
(cytotoxic T lymphocytes) recognize and destroy the
infected cells
44
45. Reference
• Roitt, I., Brostoff, J., Male, D. Immunology.
1993. Mosby-Year Book Europe Limited,
London. 3rd Ed.
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