IMMUNOLOGY

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IMMUNOLOGY

• Sherko A Omer
• MB ChB, MSc., PhD

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LYMPHOID ORGANS AND TISSUES
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LYMPHOID ORGANS AND TISSUES

• In the primary organs (thymus and mammalian bursa
  equivalent) lymphopoiesis occurs, lymphoid stem
  cells differentiates to lymphocytes which acquire
  several surface markers and become able to
  discriminate between self and non self.
• In secondary lymphoid organs (lymph nodes,
  spleen, MALT), there are the environments in
  which the lymphocytes can interact with
  immunogens to induce an immune response.

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THYMUS
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THYMUS
Thymus is composed from cortical and medullary
epithelial cells, stromal cells, interdigitating
cells and macrophages. These accessory cells
are important in the differentiation of the
immigrating T cell precursors and their
education (positive and negative selection)
prior to their migration into the secondary
lymphoid tissues.
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THYMUS
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THYMUS
The thymus has an interactive role with the
endocrine. Thymic epithelial cells produce the
hormones thymosin and thymopoietin and in concert
with cytokines (such as IL-7) are probably
important for the development and maturation of
thymocytes into mature T cells.
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THYMIC SLECTION
Positive selection of cells whose receptor binds
MHC molecules. Negative selection and death of
cells with high-affinity receptors for self-MHC
or self-MHC self-antigen.
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THYMIC DISORDERS

• Impairment of thymus development leads to immune
  deficiency such as in DiGeorge Syndrome (cellular
  immune deficiency, congenital cardiac anomalies,
  abnormal fascies and hypoparathyroidism).
• Patient with systemic lupus erythematosus (SLE)
  or myasthenia gravis may show lymphoid
  hyperplasia or thymoma.

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MAMMALIAN EQUIVALENT BURSA

• Not identical organ such as bursa
• Gut associated lymphoid tissue (appendix, Peyers
  patches) histologically resembles the birds
  bursa.
• In human foetal liver and bone marrow are
  equivalent to bursa where B cell differentiation
  and maturation take place.

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MAMMALIAN EQUIVALENT BURSA

• The bone marrow gives rise to all of the lymphoid
  cells that migrate to the thymus and mature into
  T cells, as well as to the major population of
  conventional B cells.
• B cells mature in the bone marrow and undergo
  selection for non-self before making their way to
  the peripheral lymphoid tissues.

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LYMPH NODES
Encapsulated round or bean-shaped structures that
are present in high numbers at junctions of major
lymphatic tracts. In the resting state lymph
nodes are about 1-25 mm in length but they
enlarges during infection and when they are
invaded by malignancy.
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Structure of a lymph node
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LYMPH NODES
Within the cortex there are few aggregations of
predominately B cells with mIgD (primary
follicles). Activation of B cells in the
germinal centre of primary follicles results in
division of B cells to generate secondary
follicles that contain mostly IgG-bearing B cells.
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LYMPH NODES
In secondary follicles stimulated B cells matures
to plasma cells in cords in the medulla where
they secrete Immunoglobulin. In the deep cortex
or paracortical area, T lymphocytes are the
predominant cell population. DCs are also present
in this area, where they present antigen to T
lymphocytes.
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LYMPH NODES
The medulla, less densely populated, is organized
into medullary cords draining into the hilar
efferent lymphatic vessels. Plasmablasts can be
easily identified in the medullary cords.
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SPLEEN
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SPLEEN
The spleen is the major site of immune responses
to blood-borne antigens. The spleen is also an
important filter for the blood. Its red pulp
macrophages clear the blood of microbes and other
particles. The spleen is the major site for the
phagocytosis of antibody-coated (opsonised)
microbes.
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MUCOSAL-ASSOCIATED LYMPHOID TISSUES
MALT encompass the lymphoid tissues of the
intestinal tract, genitourinary tract,
tracheobronchial tree, and mammary glands. MALT
are unencapsulated and contain both T and B
lymphocytes, the latter predominating.
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MUCOSAL-ASSOCIATED LYMPHOID TISSUES

• GALT (Gut-associated lymphoid tissues) is the
  designation proposed for all lymphatic tissues
  found along the digestive tract.
• Three major areas of GALT that can be identified
• Tonsils
• Peyers patches, located on the submucosa of the
  small intestine and the appendix
• Scanty lymphoid tissue is present in the lamina
  propria of the gastrointestinal tract

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MUCOSAL-ASSOCIATED LYMPHOID TISSUES
Lymphoid complexes along the gastrointestinal
tract volume of the rings indicates the relative
amount of lymphoid tissue.
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MUCOSAL-ASSOCIATED LYMPHOID TISSUES
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MUCOSAL-ASSOCIATED LYMPHOID TISSUES
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MUCOSAL-ASSOCIATED LYMPHOID TISSUES
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MUCOSAL-ASSOCIATED LYMPHOID TISSUES
M cells, present largely in the dome epithelia
of Peyers patches, particularly at the
ileum. These cells take up small particles,
virus, bacteria, and deliver them to sub-mucosal
macrophages, where the engulfed material will be
processed and presented to T and B lymphocytes.
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LYMPHOCYTE RECIRCULATION
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LYMPHOCYTE RECIRCULATION
The cells of the lymphatic system circulate
continuously and reach all parts of the body with
a few exceptions (vitreous body, brain,
testicles). They reach the lymph nodes, skin,
and intestine via a specialized endothelium of
post-capillary venules, the so-called high
endothelial venules (HEV). The cells of this
endothelium are much higher than normal
endothelial cells.
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LYMPHOCYTE RECIRCULATION
In response to certain chemotactic factors,
lymphocytes migrate to the underlying tissue
(diapedesis). The lymphatic cells re-enter the
circulation through efferent lymph vessels that
merge into the thoracic duct. The lymphocytes
enter the spleen via arterioles and sinusoids and
exit the organ via the splenic vein
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LYMPHOCYTE RECIRCULATION
Mucosal B cells circulate between different
segments of the MALT, including the GALT, the
mammary gland-associated lymphoid tissue, and the
lymphoid tissues associated with the respiratory
tree and urinary tract. T cells migrate to lymph
nodes. The selectivity in migration is due to
presence of homing receptors on lymphocytes
surface that can bind with the endothelial cells
in HEV. CD44, on lymphocytes and endothelium are
important in these routings.
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LYMPHOCYTE RECIRCULATION
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ADHESIOIN MOLECULES
Surface molecules called cell adhesion molecules
(CAMs) that interact with ligands, including
other CAMs, expressed on the membrane of T and B
lymphocytes. Three main families of CAMs have
been defined Selectins Immunoglobulin
superfamily of CAMs Integrins
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MAJOR HISTOCOMPALABILITY COMPLEX
Mammalian species possess a tightly linked
cluster of genes, the major histocompatibility
complex (MHC). The MHC is referred to as the
Human Leukocyte Antigen (HLA) complex in humans
and as the H-2 complex in mice. MHC are
important in graft rejection.
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MAJOR HISTOCOMPALABILITY COMPLEX
Class I MHC genes encode glycoproteins expressed
on the surface of nearly all nucleated cells the
major function of the class I gene products is
presentation of peptide antigens to TC cells.
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MAJOR HISTOCOMPALABILITY COMPLEX
Class II MHC genes encode glycoproteins expressed
primarily on antigen-presenting cells
(macrophages, dendritic cells, and B cells),
where they present processed antigenic peptides
to TH cells. Class III MHC genes encode, in
addition to other products, various secreted
proteins that have immune functions, including
components of the complement system and molecules
involved in inflammation
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MAJOR HISTOCOMPALABILITY COMPLEX
Class II MHC genes encode glycoproteins expressed
primarily on antigen-presenting cells
(macrophages, dendritic cells, and B cells),
where they present processed antigenic peptides
to TH cells. Class III MHC genes encode, in
addition to other products, various secreted
proteins that have immune functions, including
components of the complement system and molecules
involved in inflammation.
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INHERITANCE OF MHC
An individual inherits one haplotype from the
mother and one haplotype from the father. The
alleles are codominantly expressed that is, both
maternal and paternal gene products are expressed
in the same cells. The chance of two siblings to
have both haplotypes identical is 25, one
haplotype identical is 50 and zero haplotype
identical is 25, provided the chance of
recombination is neglected.
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INHERITANCE OF MHC
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INHERITANCE OF MHC
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MHC CLASS I MOLECULE
45-kiloDalton (kDa), ? chain associated
noncovalently with a 12-kDa ?2-microglobulin
molecule. The ? chain is a transmembrane
glycoprotein encoded by polymorphic genes within
the A, B, and C regions of the human HLA
complex. ?2-Microglobulin is a protein encoded
by a highly conserved gene located on chromosome
15.
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MHC CLASS I MOLECULE
Class I molecules are present on the surface of
all nucleated cells and on platelets. CD8 cells
can recognize antigen only when this antigen is
associated with self class I molecule on the
surface of cells (virally infected, tumour cell)
this is called MHC restriction.
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MHC CLASS I MOLECULE
CD8 cells cannot recognize antigen alone or when
these antigens are associated with different MHC
class I molecules. Class I molecules are called
major transplantation antigens because they are
recognized by host during graft rejection.
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MHC CLASS II MOLECULE
Consist of two different polypeptide chains, a
33-kDa ? chain and a 28-kDa ? chain. ?1 and ?2
domains in one chain and ?1 and ?2 domains in the
other. The membrane-proximal ?2 and ?2 domains,
like the membrane-proximal ?3/?-2-microglobulin
domains of class I MHC molecules, bear sequence
similarity to the immunoglobulin-fold structure.
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MHC CLASS II MOLECULE
The membrane-distal portion of a class II
molecule is composed of the ?1 and ?1 domains and
forms the antigen binding cleft for processed
antigen.
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MHC CLASS II MOLECULE
Class II molecules are present on surface of B
cells, APCs, activated T cells, and in some
conditions on resting T cells, thyroid
cells. CD4 cells can recognize antigens only
when the antigen is associated with self MHC
class II molecule and these molecules are
responsible for Graft Versus Host reaction (GVH).
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DETECTION OF HLA MOLECULES

• HLA molecules can be identified on lymphocytes
  by
• Lymphocytotoxicity test (antibody against HLA
  molecules with added complement) for detection of
  both class I and II molecules.
• Molecular methods.

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HLA AND DISEASE ASSOCIATION
Most of the diseases associated with HLA are
autoimmune in nature some are infectious (e.g.,
malaria). Diseases are of unknown aetiology and
pathophysiology and with a hereditary pattern of
distribution. The diseases are related to
immunological abnormalities.
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HLA AND DISEASE ASSOCIATION

• The explanation of HLA disease relation
• HLA molecules may act as receptors for
  etiological agents
• HLA may act as selective sites for antigen and
  then the cell present this antigen to induces an
  immune response
• The causative agent may mimic HLA molecule

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HLA AND DISEASE ASSOCIATION
Disease Associated HLA Allele Relative Risk
Ankylosing spondylitis B27 90 Rheumatoid
arthritis DR4 10 SLE DR3 5 Multiple
sclerosis DR2 5 ID diabetes
mellitus DR4/DR3 20 Myasthenia gravis
DR3 10
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ANTIGEN AND IMMUNOGEN
Immunogenicity and antigenicity are related but
distinct Immunogenicity is the ability to
induce a humoral and/or cell-mediated immune
response. Antigenicity is the ability to combine
specifically with antibodies and/or TCR.
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ANTIGEN AND IMMUNOGEN
Small molecules called haptens, are antigenic but
incapable, by themselves, of inducing a specific
immune response. Haptens lack immunogenicity. Alt
hough a substance that induces a specific immune
response is usually called an antigen, it is more
appropriately to be called an immunogen
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IMMUNOGENICITY DETERMINANTS
Foreignness, immune system can distinguish self
from nonself (foreign) and only reacts against
nonself molecules to induce an immune
response. Molecular size, potent immunogens have
molecular weight more than 100 000 Da while
molecules with less than 10 000 Da are none or
weak immunogens.
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IMMUNOGENICITY DETERMINANTS
Chemical complexity, proteins and
polysaccharides are among the most potent
immunogens, small polypeptide chains, nucleic
acids, and even lipids can be immunogenic
provided given in an appropriate
circumstances. Susceptibility to antigen
processing and presentation, Macromolecules that
cannot be degraded and presented with MHC
molecules are poor immunogens.
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IMMUNOGENICITY DETERMINANTS
Genetic background of the host, different animal
species and different strains of one given
species may show different degrees of
responsiveness to a given immunogen. Dosage and
method of antigen administration, an insufficient
dose will not stimulate an immune response either
because it fails to activate enough lymphocytes
or it may induce tolerance, conversely, an
excessively high dose can also induce tolerance
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IMMUNOGENICITY DETERMINANTS
The method of immunogen administration, a given
dose of antigen may elicit no detectable response
when injected intravenously, but a strong immune
response is observed if injected intradermally.
The presence of DCs in the dermis may be a
critical factor determining the enhanced immune
responses when antigens are injected
intradermally. Use of Adjuvants, adjuvants when
administered along with immunogens enhance the
specific response.
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ADJUVANTS
Substances that, when mixed with an immunogen and
injected with it, enhance the immunogenicity of
that immunogen. Adjuvants are often used to
boost the immune response when immunogenicity to
a material is low or when only small amounts of
an immunogen/antigen are available. Examples of
adjuvant are Freunds complete adjuvant (mineral
oil, lanolin and killed Mycobacteria) used in
animal experiments and Aluminum hydroxide used in
vaccines
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ADJUVANTS

• Adjuvants augment the immune response appear to
  be through one or more of the following effects
• Antigen persistence is prolonged
• Co-stimulatory signals are enhanced
• Local inflammation is increased
• The nonspecific proliferation of lymphocytes is
  stimulated

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HAPTEN

• Chemical coupling of a hapten to a large protein,
  called a carrier, yields an immunogenic
  hapten-carrier conjugate
• Animals immunized with such a conjugate produce
  antibodies specific for
• Hapten determinant
• Unaltered epitopes on the carrier protein
• New epitopes formed by combined parts of both
  the hapten and carrier

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EPITOPE (Antigenic Determinate)
Sites on or within the antigens with which the
antibody or TCR reacts. Structurally an epitope
may be in the form of 4-5 amino acids or sugar
resides in a linear or conformational form. The
numbers of epitopes of an antigen are vary and
related to the size of the antigen, the sum of
the epitopes determines the specificity of that
antigen.
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EPITOPE
T cells can be trigged by immunogen with one
epitope while B cells need at least two epitopes
to be triggered. Diagram of sperm whale
myoglobin showing locations of five sequential
(linear) epitopes
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EPITOPE
Linear sequence of peptides recognized by T cells
Antigenic determinants (epitopes) required by
antibodies
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T-DEPENDENT AND T-INDEPENDENT ANTIGEN
B cells respond to antigen either in a T-
dependent (needs TH) or a T- independent
fashion. T- independent antigens have high
molecular weight with linearly arranged repeating
antigenic determinants and are highly resistant
to degradation by body enzymes Examples of
T-independent antigens are Pneumococcal
polysaccharides, Escherichia coli
lipo-polysaccharides, and polyvinyl pyrolidine.
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T-DEPENDENT AND T-INDEPENDENT ANTIGEN
Most natural antigens are TD and requires antigen
processing by antigen presenting cells (APCs).
Feature T- independent antigen T- dependent antigen
Immune response Weak Strong
Secondary response IgM IgM or IgG
Isotype switching Negative Positive
Memory cell induction Poor Good
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EXOGENOUS ENDOENOUS ANTIGEN
Exogenous (foreign) from outside Exogenous
antigen include microbial, environmental
(pollens and pollutants) and medications. Immune
system try to eliminate foreign antigens, but in
some instances, the immune response itself may
have a deleterious effect, resulting in
hypersensitivity or in autoimmune disease.
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EXOGENOUS ENDOENOUS ANTIGEN
Endogenous antigens are part of self, the immune
system is usually tolerant to them. The
response to self-antigens may have an important
role in normal catabolic processes (i.e.,
antibodies to denatured IgG may help in
eliminating antigen-antibody complexes from
circulation antibodies to oxidized low density
lipoprotein (LDL) may help in eliminating a
potentially toxic lipid .
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EXOGENOUS ENDOENOUS ANTIGEN
However, the loss of tolerance to self-antigens
can also result in autoimmune diseases.
Endogenous antigens that distinguish one
individual from another within the same species
and are termed alloantigens, these elicit
immune responses when cells or tissues of one
individual are introduced into another.
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EXOGENOUS ENDOENOUS ANTIGEN
Separate antigen-presenting pathways are
utilized for endogenous and exogenous antigens.
The mode of antigen entry into cells and the
site of antigen processing determine whether
antigenic peptides associate with class I MHC
molecules in the rough endoplasmic reticulum or
with class II molecules in endocytic compartments
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MITOGEN
A substance which causes DNA synthesis and blast
transformation, mitogen has also
immunostimulatory effects by increasing expansion
of B and/or T cells. Concanavalin A (Con A) T
cells Phytohaemagglutinin (PHA) T
cells Pokeweed mitogen (PWM) T and B cells
Lipopolysaccharide (LPS) B cells