Presentation outline
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Critical functions of immune system
Dual nature
Characteristics
Functions
Features
Mechanisms of immune response
Types of immune systems
The immune system is
crucial to human survival.
• The immune system consists of a complex
network of organs, cells, and molecules
which work together to defend the body
from disease causing organisms (bacteria,
viruses, fungi, and parasites).
• The purpose of the immune system is to
maintain homeostasis, which includes
protecting the body from pathogens and
toxins that could disrupt the homeostasis.
The Body’s Defenses:
The Immune System
Vertebrate Immune Mechanisms
• Innate Immunity
• Acquired Immunity
• Cell-Mediated Response
• Humoral Response
• Lymphatic Tissues
Multilayered and dual nature of
the Immune System
The architecture of the immune system is multi-layered, with
defenses on several levels.
Several barriers (external to internal /simple to complex) provided
at many levels of infection.
The immune system has a series of dual natures:
Self/ non-self recognition
Natural / adaptive
Cell mediated / humoral
Passive / active
Primary / secondary
• Both systems consist of a multitude of cells and molecules that
interact in a complex manner to detect and eliminate pathogens.
Characteristics of the immune
response
1. Specificity: the ability to distinguish between
antigens
2. Adaptiveness: the ability to respond to previously
unseen molecules.
3. Recognition of self and non-self : the ability to
recognize and respond to molecules that are
foreign or ‘non-self ‘and the molecules that are
‘self’.
4. Memory: the ability to recall previous contact with a
foreign molecule and respond to it in a learned
manner.
Functions of immune system
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The immune system like any organization, has
members that perform different functions to
accomplish a common goal.
Provides defenses against pathogens
Removes dead or worn out cells like RBCs
Identifies and destroys abnormal cancer cells
Protects from autoimmune diseases
Rejects tissues cells of foreign antigens .
Main features of Immune system :
Pattern recognition
(anomaly detection, noise tolerance),
distributed, No central point of control
diversity,
learning,
memory,
redundancy,
robustness,
feature extraction,
multilayered, and
adaptive.
Mechanism of immune response
• The immune system specifically recognizes and
selectively eliminates pathogens.
• There are two critical steps in the immune response:
1. Detection events
2. Elimination events
• The detection and elimination of pathogens depend
upon the chemical bonding between receptors on the
surface of an immune cell and epitopes found on the
surface of a pathogen.
• The complementary receptor – epitope binding
( monospecificity) activates a complex system of
signalling that mediates the immune response.
Mechanism of immune response
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Immune recognition phase is critical in the
normal functioning of the system.
• This is accomplished by three sets of antigen
binding molecules :
1. T- cell antigen receptors (TCR)
2. Class I and class II molecules of the MHC
3. B-cell antigen receptor (BCR,
immunoglobulin).
• The effector phase is mediated by a variety of
cells and soluble factors.
Immune System
Two types of immune system
􀁺 Innate immune system
􀁺 Adaptive immune system
Main Types of Immunity in Man
Immunity
Innate
immunity
Adaptive
immunity
Natural
Artificial
Passive (maternal)
Passive (Serotherapy)
Active (Infection)
Active (Vaccination)
Innate Immunity
• responds immediately
• protects the body from all foreign substances
• functions on two levels
• first line of defense – prevents entry of
pathogens
• mechanical barriers
skin, mucosa, and secretions
• second line of defense – inhibits spread of
pathogen
• antimicrobial proteins
• phagocytes and other cells
• inflammation
Three Lines of Defense
1st Line – barriers at portals of entry
primarily inborn and nonspecific
2nd Line – protective cells and chemicals
come into play when barriers are breached
primarily inborn and nonspecific
3rd Line – antibodies and cytotoxic cells
provides long-term immunity after encounters
primarily acquired and specific
Lines of defense
• 1st lines of defense are the physical barriers
which include the skin, urine, tears, ciliary
elevator, mucosal membrane, etc.
• 2nd lines of defense are the macrophage
system, complement, fever, interferon and
inflammation.
• 3rd lines of defense are the specific system
also known as acquired or adaptive
immunity. The specific system consists of B
cells (humoral), and T cells (cell-mediated).
Physical and Chemical
Defenses
First line of defense-1
• The first line of defense are barrier tissues
such as the skin that stop the entry of
pathogens into our bodies.
• If these barrier layers are penetrated,
cells like macrophages and neutrophils
engulf foreign organisms and kill them
without the need for antibodies
First Line of Defense-2
• Skin provides an almost impenetrable
biological barrier.
• Lysozyme is an enzyme found in tears and
saliva that can break down foreign agents .
• The clotting of blood near open wounds
prevents an open space for antigens.
• Mucus and cilia found in the nose and throat
can catch foreign agents then sweep them
outside via coughing, sneezing and vomiting.
• The cell wall of plants consists of fibrous proteins
which provide a barrier to potential parasites
(antigens).
A second line of defense-1
• A second line of defense is the specific or adaptive
immune system which may take days to respond to a
primary invasion
• The production of antibodies and cell-mediated responses
may occur in which specific cells recognize foreign
pathogens and destroy them.
• The response is often more rapid because of the activation
of memory B and T cells.
• cells of the immune system interact with one another by a
variety of signal molecules.
• These signals may be proteins such as lymphokines,
cytokines and chemokines which stimulate cells of the
immune system.
Second Line of Defense-2
• Second lines of defense deal with antigens that
have bypassed the first lines of defense.
• Interferons are a family of proteins that are
released by a cell that is under attack by an
antigen.
• One method of attacking antigens is by
phagocytosis , where the contents of the
antigen are broken down by molecules called
phagocytes.
Adaptive immunity
• The second level of defence increases in
strength and effectiveness with each
encounter. The foreign agent is recognised in a
specific manner and the immune system
acquires memory towards it.
• The first encounter with an antigen is known as
the primary response. Re-encounter with the
same antigen causes a secondary response
that is more rapid and powerful
Adaptive immunity
• As response produce antibodies to specific
infections
Some features
– Memory
• Each successive encounter with the same
pathogen improve the response
– Clonal selection
• Select the best and matured immune cells to
defend the organism against infections
• Whenever T cells and B cells are activated,
some become "memory" cells. The next time
that an individual encounters that same
antigen, the immune system is primed to
destroy it quickly. This is active immunity.
• Long-term active immunity can be naturally
acquired by infection or artificially acquired
by vaccines
• Short-term passive immunity can be
transferred artificially from one individual to
another via antibody-rich serum;
Active and Passive Acquired
Immunity
• Active immunity
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Conferred by recovery from infectious disease
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Depends on persons own immune system.
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May be acquired artificially with vaccines.
• Passive immunity
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transferred from person to person through transfer of antibodies.
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Natural instances include antibodies passing across placenta
from mother to fetus (provides temporary protection) or through
the milk.
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Transferred artificially. E.g. antibody injections for rabies, anti
rhesus factor, snake venom, botulinun toxin.
• Lymphatic vessels are responsible for flow of lymph within
the lymphoid system
• Plasma constantly leaks out of capillaries to deliver oxygen
and nutrients to cells of the surrounding tissue. Once in the
tissue, the plasma is now called tissue fluid .
• While most of this tissue fluid re-enters capillaries and is
returned directly to the bloodstream, some fluid enters
lymph vessels as lymph .
• LYMPH- flows from the intercellular tissue spaces into
lymphatic capillaries and then into a series of larger
collecting vessels called lymphatic vessels. Lymphatic
vessels converge into the thoracic duct, which returns the
fluid to the circulatory system by dumping into the left
subclavian vein.
Overview of blood and lymph system and how
leukocytes travel from one system to another
Thoracic
duct
Lymph carrying antibodies
and immune cells collect in
thoracic duct where the
lymph empties back into the
blood circulatory system
Site of exchange between the
blood and lymph systems
Immune cells travel back and forth from
the blood and lymph circulatory systems
and interact with extra-vascular tissues in
the process--extravasation
muscle
Lymphatic vessels.
The organs of the
immune system
Kinds of lymphoid Organs
Primary/central lymphoid organs
– bone marrow,
– thymus
Secondary /peripheral lymphoid tissue
– lymph nodes,
– spleen,
– mucosa-associated lymphoid tissue
Kinds of Lymphoid Organs
The Lymphoid
Organs
Primary or
Central organs
1.Bone marrow
2.Thymus gland
3.Bursa of fabricius
Secondary or
Peripheral organs
1.Lymp nodes
2.Spleen
3.Diffuse lymphoid tissues
• The bone marrow and the thymus constitute
the primary lymphoid organs.
• Both B-lymphocytes and T-lymphocytes are
produced from stem cells in the bone marrow.
B-lymphocytes mature in the bone marrow
while T-lymphocytes migrate to the thymus
and mature there.
• After maturation, both B-lymphocytes and Tlymphocytes circulate through and
accumulate in secondary lymphoid organs.
• The secondary lymphoid organs include lymph nodes
and the spleen, as well as lymphoid tissues.
• The lymphoid tissues include the tonsils, the appendix, the
Peyer's patches in the lining of the small intestines (gutassociated lymphoid tissue or GALT),
• The lymphoid tissue found beneath the mucous
membranes of the bronchi (bronchial-associated
lymphoid tissue or BALT),
• The lymphatic tissue found in the mucous membranes
(mucosa-associated lymphoid tissue or MALT)
• The lymphatic tissue found beneath the skin (skinassociated lymphoid tissue or SALT).
Primary and Secondary
Lymphoid organs
Primary/central lymphoid organs
BONE MARROW is a central organ where
all the immune cells are born and only B
cells mature (process termed as B
lymphopoiesis).
THYMUS is the other central organ in
which T cells mature (T lymphopoiesis).
The bone marrow
Bone marrow is a spongy tissue found within
bones.
All blood cells are produced within the bone
marrow.
The bone marrow of an adult produce about
three million red blood cells and 120,000 white
blood cells every second.
All these cells originate from a small population
of stem cells, which may be as few as one in a
million of the bone marrow cells.
Thymus
•The thymus is an organ located in the upper
chest.
•Immature lymphocytes leave the bone marrow
and find their way to the thymus where they are
“educated” to become mature T-lymphocytes.
Thymus
Flat, bilobed organ situated above the heart and below
the thyroid gland.
Each lobe is surrounded by a capsule and is divided
into lobules.
The lobules are separated from each other by strands
of connective tissue called trabeculae.
Each lobule is organized into two compartments: the
cortex (outer compartment) the medulla (inner
compartment).
• Cortex and Medulla are both crisscrossed by a
network of stromal cells composed of:
• loosely packed thymic epithelial cells
• interdigitating dendritic cells
• macrophages
Thymus
• In the cortex, the network is densely packed with
thymocytes. Cells are less dense in medulla.
• In medulla, the epithelial cells are more visible and
HASSAL's Corpuscles are present (Function Unknown).
• The thymus is at its largest relative size at birth and its
largest actual size is at puberty.
• After puberty the thymus begins to shrink. In elderly
individuals it is usually less than 3 grams in weight.
• There is relationship between aging and a decline in
immune responsiveness.
• Stress can also result in shrinkage of the thymus.
Thymus
• Precursor T cells enter thymus from the blood
(there are no afferent lymphatic vessels] and
mature into functional T lymphocytes.
• However, the vast majority of these cells are
destined to die due to (APOPTOSIS) programmed cell death.
• The T cells (thymocytes) go through a selection
process in the thymus based upon the TcR that
they possess.
Process of thymic education
• The complete process of thymic education is a two-step
process in which Thymic cortical epithelial cells function as
the effector cells in a process known as POSITIVE
SELECTION.
• In positive selection, T cells which bear a TcR which can
bind SELF-MHC are selected to survive and proliferate.
Cells which are not positively selected are triggered to
undergo APOPTOSIS .
• Positively selected thymocytes must go through a second
phase of selection known as NEGATIVE SELECTION.
• Functionally, during negative selection any T cell that is
presented antigen + MHC within the thymus is triggered to
undergo APOPTOSIS.
• Most importantly: thymic macrophages and dendritic cells
serve as APCs in the process of negative selection
Thymic hormones
• Stromal cells secrete soluble factors
which are important in T cell maturation.
• Some examples include:
alpha- 1- thymosin
beta - 4- thymosin
thymopoietin
thymulin
Lymph nodes
• Small, bean-shaped lymph nodes are laced
along the lymphatic vessels, with clusters in the
neck, armpits, abdomen, and groin.
• Each lymph node contains specialized
compartments where immune cells
congregate, and where they can encounter
antigens.
• Immune cells and foreign particles enter the
lymph nodes via incoming lymphatic vessels.
All lymphocytes exit lymph nodes through
outgoing lymphatic vessels.
Lymph Node
The regions of Lymph node
3 regions to the lymph node:
• Cortex (outermost layer- contains mostly B lymphocytes,
plus both follicular dendritic cells and macrophages all
arranged in clusters called primary follicles).
• Following antigenic stimulation the primary follicles
become secondary follicles consisting of concentric rings of
densely packed lymphocytes and central lymphocytes,
macrophages, and dendritic cells.
• The GCs (germinal centers) contain large proliferating B
lymphocytes and plasma cells interspersed with
macrophages and dendritic cells.
The regions of Lymph node
• The GC is a site of intense B-Cell activation and
differentiation into plasma cells and memory cells.
• Paracortex - (layer just beneath the cortex) This region is
sometimes called the T dependent region of the lymph
node.
It is an area populated with T lymphocytes and also
interdigitating dendritic cells. It is an important site for T
cell activation by these APCs.
• Medulla- inner most region, more sparsely populated by
cells. Many of the cells are plasma cells, activated Th and
Tc are also present. In addition, there is a high
concentration of immunoglobulin in this region due to the
large population of plasma cells.
Swelling of lymph nodes
• This huge increase in the number of lymphocytes is
due to:
• proliferation of lymphocytes within the lymph node
• influx of lymphocytes from the circulatory system
• -Clearly, the concentration of lymphocytes in the
lymph nodes can increase greatly following antigenic
stimulation accounting for "swelling of lymph nodes".
The spleen
• The spleen is the largest of the lymphoid
organs.
• 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 work, and serves as a meeting
ground where immune defenses confront
antigens.
Spleen
Spleen
Spleen
• The spleen is adapted to filter the blood, it
responds therefore to systemic infections.
• It is surrounded by a capsule, which sends
trabeculae into the interior to form a
compartmentalized structure. There are two
types of compartments [Red Pulp and White
Pulp].
Spleen
• Red Pulp- Network of sinusoids populated with
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macrophages and numerous erythrocytes. Site where old
RBCs are destroyed and removed.
White Pulp- Surrounds the splenic arteries, forming a
periarteriolar lymphoid sheath (PALS) populated mainly
by T lymphocytes.
Clusters of B lymphocytes in the White Pulp form primary
follicles occupying a more peripheral position.
Upon antigenic challenge, these primary follicles
develop into characteristic secondary follicles
containing germinal centers.
The spleen is NOT supplied by afferent lymphatics.
Functions of the Spleen
• The spleen functions both as immune and
hematopoietic systems.
The immune functions include:
• proliferation of lymphocytes
• production of humoral antibodies
• removal of macromolecular antigens from the blood
The hematopoietic functions include:
• formation of blood cells in fetal life
• removal and destruction of senile, damaged and
abnormal RBCs and platelets
• retrieval of the iron from hemoglobin
• storage of blood in some species (not humans)
Lymphoid tissues
• Clumps of lymphoid tissue are found in
many parts of the body, especially in the
linings of the digestive tract and the
airways and lungs—territories that serve
as gateways to the body. These tissues
include the tonsils, adenoids, and
appendix.
Peyer's patches
Peyer's patches are areas of lymphoid tissue
located in the wall of the intestine, and in some
mammalian species such as sheep, cattle and
rabbits,
Peyer's patches have a function similar to the
bursa of fabricius of birds and bone marrow of
other mammals: B cell differentiation and
maturation.
• Actually: Two types of Peyer's patches seem to
occur: one with primary lymphoid function and
another with secondary lymphoid function.
Functions of lymphatic organs
1. Generate immuno-competant
lymphocytes.
2. Concentrate antigens into lymphoid
organs.
3. Circulate lymphocytes through
lymphoid organs, exposing antigen to a
variety of lymphocytes.
4. Deliver antibodies and effector T cells to
the blood and tissues.
Kinds of Adaptive immunity
•Adaptive immune responses are
actually reactions of the immune
system to antigens.
• There are two types of adaptive
immune responses:
• Humoral and cell mediated.
Adaptive Humoral immunity
• During humoral immune responses, proteins
called antibodies, destroy antigens, appear in
the blood and other body fluids.
• Humoral immune responses resist invaders that
act outside of cells, such as bacteria and toxins
(poisonous substances produced by living
organisms).
• Humoral immune responses can also prevent
viruses from entering cells.
Adaptive cellular immunity
• During cell-mediated immune responses, cells
that can destroy other cells that are either
infected with, or producing, a specific antigen.
• Cell-mediated immune responses resist
invaders that reproduce within the body cells,
such as viruses.
• Cell-mediated responses may also destroy
cells making mutated (changed) forms of
normal molecules, as in some cancers.
Immunity is a coordinated effort
by the body.
• It was previously believed that the immune system
functioned independently in the body.
• It’s now known that there are several links between the
immune, nervous and endocrine systems.
• Certain immune responses trigger endocrine &/or
nervous activity.
• Lymphocytes & macrophages are responsive to signals
from the nervous system and certain endocrine glands.
• Therefore, immunity is the job of the entire body not just
one system.
About the presenter
• Dr.B.Victor is a highly experienced postgraduate biology
teacher, recently retired from the reputed educational
institution St. Xavier’ s College, Palayamkottai, India627001.Presently HOD of Biotech at Annai Velankanni
college, Tholayavattam. K .K Dist
• He was the dean of sciences and assistant controller of
examinations of St. Xavier’s college.
• He has more than 32 years of teaching and research
experience and has guided more than 12 Ph. D scholars.
• He has taught Immunology and biochemistry to
graduate and post graduate students.
• Send your comments to : bonfiliusvictor@gmail.com