ADAPTIVE IMMUNITY
By
Prof Dr Asghar Javaid

ADAPTIVE IMMUNITY
• The next line of host defense is the adaptive
immune system, which is composed of lymphocytes
(also called lymphoid cells) and their secreted
factors.
• The immune response is specifically tailored
against different microbes.

ADAPTIVE IMMUNITY
• This is achieved by first generating an enormous
number of diverse lymphocytes, each with a unique
antigen specificity.
• Before they see their antigen, these lymphocytes
are called naïve .

ORIGIN OF LYMPHOID CELLS
• All white and red blood cells originate from stem
cells in the fetal liver and yolk sac during embryonic
life and in the bone marrow after birth
• . The common lymphoid progenitor is a type of
stem cell that gives rise to lymphocytes of the
adaptive immune system, including B cells and T
cells

ORIGIN OF LYMPHOID CELLS
• The common lymphoid progenitor is also the source of
innate lymphocytes, such as natural killer (NK) cells.
• The process by which common lymphoid progenitors
develop into lymphocytes depends on cytokines, and
mutations in the genes encoding the receptors of
these cytokines are often the cause of severe
combined immunodeficiency, a complete absence of
mature lymphocytes

ORIGIN OF LYMPHOID CELLS
• The ratio of T cells to B cells is approximately 3:1.
• Often T cells are named by markers we can detect
on their cell surface, called “cluster of
differentiation” (CD) markers:
• helper T cells are CD4-positive (CD4+), whereas
cytotoxic T cells are CD8-positive (CD8+).

ORIGIN OF LYMPHOID CELLS
• All vertebrates produce enormously diverse pools of
antigen receptors;
• in humans, this pool is estimated to comprise 100
million different specificities, protecting us from
millions of potential pathogens.

T-cell Development
• Prior to entering the thymus, stem cells lack antigen
receptors and lack CD3,CD4,and CD8 proteins on
the surface
• The stem cell which initially express neither CD4
nor CD8(double negative) ,first differentiate to
express both CD4 and CD8(double positive) and
then proceed to express either CD4 or CD8

T-cell Development
• A double positive cell will differentiate into a CD4-
positive cell if it contacts a cell bearing class II MHC
proteins but will differentiate into a CD8-positive
cell if it contacts a cell bearing class I MHC proteins

T-cell Development
• The double negative and the double positive cells
are located in the cortex of thymus ,whereas the
single positive cells are located in medulla.
• Within thymus two very important processes
called thymic education occur

T-cell Development
• Clonal deletion (NEGATIVE SELECTION) CD4+,CD8+
cells bearing antigen receptors for self proteins are
killed by a process of programmed cell death called
apoptosis (survival of cells with TCRs that do not
recognize self antigen)

T-cell Development
• Positive selection ---- CD4+,CD8+ cells bearing
antigen receptors that do not react with self MHC
proteins are also killed (survival of cells with TCRs
that react with self MHC)

Why is this important?
• For a T cell to function, it is essential that its TCR
can interact strongly with an appropriate MHC
molecule, and positive selection guarantees that
the T cells that eventually exit the thymus have
functional MHC binding TCRs

Why is this important?
• The removal of self-reactive cells ensures that the
naïve T cells that eventually exit the thymus are not
specific for self-antigens, and this self-tolerance is
one of the key ways the immune system
discriminates between what is self and what is
foreign

Why is this important?
• Positive and negative selection remove all but the T-
cell clones that react weakly with self-peptides
presented in complexes with MHC proteins.
• The same MHC proteins that are required for initial
thymic selection of T-cell precursors later become
the critical signals for activating T cells through
their TCRs

T-cell Development
• During their passage through thymus each double
positive T-cell synthesizes a different highly specific
antigen receptor called T-cell receptor
• T-cell progenitors differentiate under the influence
of thymic hormones (THYMOSIN and
THYMOPOITEN)into T-cell subpopulations.

T-cell Development
• These cells have surface glycoproteins CD3,CD4,CD8
• T-cell are divided into two categories
CD4 or CD8
Naïve T- lymphocytes are found in blood and constitute
60% to 70%, and in T-cell zones of peripheral lymphoid
organs such as the paracortical areas of lymph nodes
and periarteriolar sheaths of the spleen.

TCR
• Each T-cell is genetically programmed to recognize a
specific cell bound antigen by means of an antigen –
specific T-cell receptor
• Each T cell expresses TCR molecules of one
structure and specificity

TCR
• TCR consists of a disulfide –linked heterodimer
made up of an α & a β polypeptide chain, each
having a variable (antigen-binding) and a constant
region, minority composed of γ and δ polypeptide
chain

TCR
• αβ TCR recognizes peptide antigens that are
displayed by MHC molecules, γ δ TCR recognizes
peptides ,lipids and small molecules without a
requirement for display by MHC proteins
• Each TCR is noncovalentally linked to CD3 (γ,δ,ε )
and dimer of ξ (zeta) chain (5 transmembrane
proteins)

CD4 lymphocytes
• 1-Help B-cell to develop into antibody –producing
plasma cells
• 2-Help CD8 T cells to become activated cytotoxic T-
cells
• 3-Help macrophage to effect delayed
hypersensitivity
Functions performed by two populations of CD4 cells

CD4 lymphocytes
Th-1 cells – a) activate cytotoxic T-cells by producing
IL-2,
b) initiate the delayed hypersensitivity
response by producing IL-2 and gamma
interferon
Th-2 cells -perform B-cell helper function by
producing IL-4 and IL-5

Comparison of Th-1cells and Th-2 cells
• IL-2 and gamma interferon
yes
• IL-4,IL-5,IL-6,IL-10 no
• Cell mediated immunity
and DTH yes
• Antibody production no
• Stimulated by IL-12 yes
• Stimulated by IL-4 no
• IL-2 and gamma interferon
no
• IL-4,IL-5,IL-6,IL-10 yes
• Cell mediated immunity
and DTH no
• Antibody production yes
• Stimulated by IL-12 no
• Stimulated by IL-4 yes

Regulator of balance between Th-1 and Th-2
cell
• IL-12 produced by macrophages ,increases Th-1
cells
• Gamma interferon produced by Th-1 cells activate
macrophage and inhibits the production of Th-2
cells
• IL-10 produced by Th-2 cells inhibit IL-12 production
by macrophages

CD8 lymphocytes
Perform cytotoxic functions
• Kill virus infected cell
• Kill tumor cells
• Kill allograft cells
They kill by two mechanisms
Release of perforins
Apoptosis

Activation of T-cells
• Ingestion of foreign protein into APC
• Cleaved small peptides in cytoplasm
• Associate with class II MHC proteins (viral with MHC-I
protein )
• Complex is transported to surface of APC
• Antigen associated with MHC-II on surface presented
to receptor on the CD4+cell ( associated with MHC-I to
CD8+ cell )

Activation of T-cells
• T-cell recognizes on polypeptides in association
with MHC molecules
• Class I proteins present endogenously synthesized
antigens (cleaved by a proteosome ) viral proteins
• Class II proteins present antigens of extracellular
organisms (cleaved within endosome)

Two Signals are required to activate T-cells
• 1-interaction of antigen and MHC protein with T-cell
receptor specific for antigen
• CD4 protein interacts with MHC class II protein
• LFA lymphocyte function associated antigen on T-
cells (both CD4+,CD8+) binds to ICAM intercellular
adhesion molecule on APC

Two Signals are required to activate T-
cells
• 2-Costimulatory signal
B-7 on APC interacts CD28 on Helper T-cell
Production of costimulatory protein depends on
activation of toll – like receptor on APC

Activation of T-cell
• Signal is transmitted by CD3 protein complex
• Activates phosphokinases, activate phospholipase C
• Which cleaves Phosphoinositide to produce inositol
triphosphate
• Opens calcium channels

Activation of T-cell
• Calcium activates calcineurin
• Calcineurin moves to nucleus
• Activation of genes for IL-2 and IL_2 receptor
• Stimulates T-cell to multiply into clone of antigen
specific T-cell

INHIBITION OF T-CELL ACTIVATION
• CTLA-4 appears on T-cell
• Binds with B-7 by displacing CD28
• Interaction inhibits T-cell activation by blocking IL-2
synthesis

Effector functions of T-cells
• Delayed Hypersensitivity antigens of intracellular organisms
e.g,Histoplasma, Mycobacterium
Mediated by CD4 cells (Th-I)
macrophage
Important cytokines gamma interferon
, macrophage activation factor,
macrophage migration inhibition factor
Role of IL-12 ---- gamma interferon axis

Effector functions of T-cells
• Cytotoxicity
• CD8 cells recognize viral antigen and MHC class I
molecules on the surface of infected cells
• Cytotoxic T cell must be activated by IL_2 produced
by helper T-cell
• Helper T-cells recognize viral antigens bound to
class molecules on an APC

Effector functions of T-cells
• Activated T-helper cell secrete IL-2 which stimulates
the virus specific cytotoxic cell to form a clone of
activated cytotoxic cell
• perforins form channels
• granzymes are proteases that degrade proteins in
cell membrane
• Activate caspases that initiate apoptosis

Cytotoxic T Cells Lyse Infected Cells

Fas-Fas Ligand interaction
• Fas is a protein displayed on the surface of many
cells
• When a cytotoxic T-cell receptor recognizes an
epitope on the surface of a target cell,FasL is
induced in the cytotoxic cells.
• When Fas and FasL interacts apoptosis of the
target cell occurs

Immune surveillance
• Many tumor cells develop new antigens on their
surface.
• These antigens bound to class I proteins are
recognized by cytotoxic T-cells which are stimulated
to proliferate by IL-2
• The resultant clone of cytotoxic T-cells can kill the
tumor cells phenomenon called Immune surveillance

Response to allograft
• Cytotoxic T-cells recognize the class I molecules on the
surface of the foreign cells
• Helper cells recognize the foreign class II molecules on
certain cells in the graft e.g,macrophages and
lymphocytes
• The activated helper cell secrete IL-2 which stimulates
the cytotoxic cell to form a clone of cells.These kill the
cells in allograft

Regulatory functions of T-cells
• Antibody production
• Cell mediated immunity
• Suppression of certain immune responses
Suppressor T-cells----CD25 ,CD4

Central Role of Helper T Cells

The Th-17 cell subset
• The Th-17 cell subset is closely related to the Th-1
cell subset but is generated by high levels of IL-1, IL-
6, and IL-23 at the time of initial activation by DCs.
• Th-17 cells express the signature transcription
factors RORC and STAT3, which are reinforced by
autocrine signaling from the cytokine IL-21.

The Th-17 cell subset
• Th-17 cells also produce the cytokines IL-17 (the
source of their name), which stimulates phagocytes
and mucosal epithelial cells to increase the
production of IL-1, IL-6, and neutrophil-attracting
chemokines.

The Th-17 cell subset
• Th-17 cells also make IL-22, which stimulates
mucosal epithelial cells to increase the production
of antimicrobial defensins and tight junction
proteins.
• Together, the cytokines from Th-17 cells and the
neutrophils they recruit defend the barrier tissues
against bacterial and fungal infections.

B cells
• During embryogenesis,B-cells precursors are
recognized in the fetal liver 7-8th week of gestation
• They do not require the thymus for maturation
• They migrate to bone marrow

B cells
• 10 -20% of circulating peripheral lymphocyte
population
• Also present in peripheral lymphoid organs such as
,lymph nodes ,spleen ,tonsils and extralymhatic
organs such as gastrointestinal tract

B cells
• Lymh node superficial cortex
• Spleen white pulp
• B –cells are located in follicles
• Maturation of B-cell has two phases
Antigen independent phase
Antigen dependent phase

B cells
Stem cell,
pro-B cell---ü chain (cytoplasm),CD10,CD19,
pre-B-cell--- ü chain +light chain,CD10,CD19
immature B-cell----CD10,CD19 ,CD20,SIg(IgM)
mature B-cell
, Activated B-cell
memory B cell
plasma cell

B CELLS
• B cells perform two important functions:
• (1) they differentiate into plasma cells that produce
antibodies (also called immunoglobulins) and
• (2) they can become long-lived memory B cells that
can rapidly respond to a reinfection.

B CELLS
• The immunoglobulin on the B-cell surface is its
antigen receptor (B-cell receptor or BCR) and the
ability of a B-cell precursor to make this antigen
receptor determines whether it is allowed to
develop into a mature B cell.

B CELLS
• B-cell precursors first arise from stem cells in the
fetal liver, but by the time of birth, these stem cells
migrate to the bone marrow, which is their main
location during childhood and adult life.
• Unlike T cells, B cells do not require the thymus for
maturation.

B CELLS
• The maturation of B cells has two phases:
• the first is the antigen-independent phase, which
consists of stem cells, pre-B cells, and B cells, and it
is during this phase that the B cell recombines its
immunoglobulin genes to make a unique antigen
receptor.

B CELLS
• For pre-B cells to differentiate into B cells, a
functional immunoglobulin must be present on the
cell surface.
• A protein called Bruton’s tyrosine kinase (BTK)
detects this immunoglobulin and signals to the cell
to continue to divide and differentiate.

B CELLS
• A mutation in the gene encoding this protein causes
X-linked agammaglobulinemia, a condition in
which cells cannot progress to the pre-B cell stage
and no antibodies are made

B CELLS
• During the second phase, which is the antigen-
dependent phase, mature B cells with functional
antigen receptors interact with antigens.
• The immunoglobulin (Ig), or BCR, of a mature B cell
is an IgM molecule with an additional region at the
end of its heavy chain that tethers it to the B-cell
surface.

B CELLS
• Approximately 109 B cells are produced each day,
but only a small fraction of these make it from the
bone marrow into the circulation, and unless they
are activated through their antigen receptors,
circulating B cells have a short life span (i.e., days
or weeks).

B cells
Markers- (BCR) IgM or IgD,
Igα/Igβ –(hetrodimer of nonpolymorphic
transmembrane proteins,)
donot bind antigen like CD3 but essential for signal
transduction through antigen receptor
MHC class I,MHC class II,B7,CD40, CD10,CD19, surface
receptors for FcγR, complement receptor(CD21)

Activation of B-cell
• B-lymphocytes may be activated by protein and non
protein antigens
• Antigens do not have to be presented to B-cells in
association with class II MHC proteins, unlike T-cells
• Antigen receptors on B-cell (IgM or IgD) recognize
many different types of molecules, such as peptides,
polysacharides,nucleic acids, and small molecules

Activation of B-cell
• Antigen is processed by APC (B-cell)
• Epitopes appear on the surface in association with
MHC class II protein
• This complex is recognised by helper T-cells with
TCR

Two costimulatory interactions
• CD28 on T-cell must react with B7 on B-cell,
required for activation of T- cell to produce IL-2
• CD40L on the T-cell must interact with CD40 on the
B-cell, required for class switching from IgM to IgG
• LFA on T-cell interacts with ICAM -1 on B-cell

Activation of B-cell
• T-cell produces cytokines (IL-2,IL-4 B-cell growth
factor,IL-5 B-cell differentiation factor).
• Cytokines stimulate growth and differentiation of
the B-cell .

Activation of B-cell
• Production of many plasma cells that produce large
amounts of immunoglobulins specific for the
epitope
• Memory cells are also produced which remain
quiescent for long periods but are capable of being
activated rapidly upon re exposure to antigen

Functions of B-cell
• They differentiate into plasma cells and produce
antibodies
• They can present antigen to helper T-cell

Immunoglobulins
Structure
Types
Characters

Antibodies
• Antibodies are globulin proteins
(immunoglobulins) that react specifically with that
antigen that stimulate their production.
• They make up about 20 % of the protein in blood
plasma.

Antibodies
• Blood contain three types of globulins, alpha, beta
and gamma, based on their electrophoretic
migration rate.
• Antibodies are gamma globulins.

Antibodies
• There are five classed of anitibodies:
• IgG, IgM, IgA, IgD and IgE.
• Antibodies are subdivided into these five classes
based on difference in their heavy chains.

Antibodies
• H chains are distinct for each of the five
immunoglobulin classes and are designated
γ,α,µ,ε,and δ.

IMMUNOGLOBULIN STRUCTURE
• Immunoglobulins are glycoproteins made up of
light (L) and heavy (h) polypeptide chains.
• The terms “light” and “heavy” refers to molecular
weight

IMMUNOGLOBULIN STRUCTURE
• light chains have a molecular weight of about
25,000 whereas heavy chains have a molecular
weight of 50,000-70,000

IMMUNOGLOBULIN STRUCTURE
• Heavy chain gene present on chromosome 14
• Light chain lambda gene are present on
chromosome 2
• Light chain kappa gene are present on chromosome
22

IMMUNOGLOBULIN STRUCTURE
• The simplest antibody molecule has a Y shape and
consists of four polypeptide chains:
• two H chains and two L chains.
• An individual antibody molecule always consists of
identical H chains and identical L chains

IMMUNOGLOBULIN STRUCTURE
• The four chains are lilnked by disulfide bonds.
• Interchain = H-H,H-L,L-L
• Intrachain =L chain has 2 bonds
γ,α,δhave 4 bonds
µ,ε have 5 bonds

IMMUNOGLOBULIN STRUCTURE
• L and H chains are subdivided into variable and
constant regions.
• The regions are composed of three dimentionally
folded repeating segment called domains.

IMMUNOGLOBULIN STRUCTURE
• An L chain consist of one variable (VL) and one
constant (CL) domain.
• Most H chains consist of one variable (VH) and
three constant (CH) domains. (IgG and IgA have
three CH domains, whereas IgM and IgE have four).

IMMUNOGLOBULIN STRUCTURE
• Each domain is approximately 110 amino acids
long.
• The constant region of the light chain has no known
biologic function.

IMMUNOGLOBULIN STRUCTURE
• The variable regions of both the light and heavy
chains are responsible for antigen-binding, whereas
the constant region of the heavy chain is
responsible for various biologic functions e.g ,
complement binding site is in the CH2 domain.

IMMUNOGLOBULIN STRUCTURE
• STRUCTURE OF THE VARIABLE REGION
• A. Hypervariable (HVR) or complementarity
determining regions (CDR)
• The variable regions of both L and H chains have
three extremely variable (hypervariable) amino acid
sequences at the amino-terminal end that form the
antigen-binding site.

IMMUNOGLOBULIN STRUCTURE
• Only 5-10 amino acids in each hypervariable region
form the antigen-binding site.
• These small portions are called complimentarity
determining regions(CDR)

IMMUNOGLOBULIN STRUCTURE
• L chains belong to one of two types, k( kappa) or λ
(lambda), on the basis of amino acid differences in
their constant regions.
• Both types occurs in all classes of immunoglobulins
(IgG, IgM etc) but any one immunoglobulin
molecule contains only one type of L chain.

IMMUNOGLOBULIN STRUCTURE
• Framework regions
• The regions between the complementarity
determining regions in the variable region are called
the FRAMEWORK REGIONS
• The remarkable specificity of antibodies is due to
these hypervariable regions.

IMMUNOGLOBULIN STRUCTURE
• Antigen-antibody binding involves electrostatic and
vander Waal’s forces and hydrogen and
hydrophobic bonds rather than covalent bonds.

IMMUNOGLOBULIN STRUCTURE
• The amino-terminal portion of each L chain
participates in the antigen-binding site.
• The amino-terminal portion of each H chains
participates in the antigen-binding site.
• The carboxy terminal portion forms the Fc
fragment, which has the biological activities.

IMMUNOGLOBULIN STRUCTURE
• If an antibody molecule is treated with a proteolytic
enzyme such as papain, peptide bond in the
“hinge” region are broken, producing two identical
fragments.

IMMUNOGLOBULIN STRUCTURE
• Fab fragments, which carry the antigen-binding site,
and
• one Fc fragment crystalisable which is involved in
placental transfer, complement fixation,
attachment site for various cells, and other
biological activities.

IMMUNOGLOBULIN STRUCTURE
• Hinge Region
This is the region at which the arms of the antibody
molecule forms a Y.
• It is called the hinge region because there is some
flexibility in the molecule at this point.

IMMUNOGLOBULIN STRUCTURE
• Oligosaccharides
Carbohydrates are attached to the CH2 domain in
most immunoglobulins.
• However, in some cases carbohydrates may also be
attached at other locations

IMMUNOGLOBULIN CLASSES

IgG
75% of the total immunoglobulin,
IgG is the major Ig in extra vascular spaces
• 1000mg/dl in blood
• Molecular wt is 150x1000
• monomer

IgG
• Each IgG molecule consists of two L chains and two
H chains linked by disulfide bonds (molecular
formula H2L2).
• Because it has two identical antigen-binding sites, it
is said to be divalent

IgG
• There are four subclasses. IgG1-IgG4, Based in
antigenic differences in the H chains and on the
number and location of disulfide bonds.

IgG
• IgG1 makes up most (65%) of the total IgG.
• IgG2 antibody is directed against polysaccharide
antigens and is an important host defense against
encapsulated bacteria.

IgG
• IgG is the predominant antibody in the secondary
response and constitutes an important defense
against bacteria and viruses.
• IgG is one of the two immunoglobulins that can
activate complement;
IgM is the other.

IgG
IgG is the only antibody to cross the placenta;
only its Fc portion binds to receptors on the surface of
placental cells .
It is therefore the most abundant immunoglobulin in
newborns.

IgG
• IgG is the immunoglobulin that opsonizes.
• It can opsonize; i.e enhance phagocytosis, because
there are, receptors for the γH chain on the surface
of phagocytes.
• IgM does not opsonize directly, because there are
no receptors on the phagocyte surface for the uH
chain.

IgG
• IgM activates complement, and the resulting C3b
can opsonize because there are binding sites for
C3b on the surface of phagocytes.

IgA
• IgA is the main immunoglobulin in secretions such
as colostrum, saliva, tears and respiratory, intestinal
and genital tract secretions.
• It prevents attachment of microorganisms e.g
bacteria and viruses, to mucous membranes

IgA
• 15% of total immunoglobulin
• Mol.wt 400,000
• Each secretary IgA molecule consists of two H2L2
units plus one molecule each of J chain and
secretary component

IgA
• The two heavy chains in IgA are α heavy chains.
• It exists as monomer and dimer form
• In serum, some IgA exists as monomeric 80%

IgA
• The secretory components is a polypeptide
synthesized by epithelial cells that provide for IgA
passage to the mucosal surface.
• It also protects IgA from being degraded in the
intestinal tract.

IgM
• IgM is the main immunoglobulin produced early in
the primary response.
• 10% of total immunoglobulin
• Mol.wt. 900,000
• 120mg/dl in blood

IgM
• It is present as a monomer on the surface of virtually all
B cells, where it function as an antigen-binding receptor.
• In serum, it is pentamer composed of five H2L2 units,
plus one molecule of joining chain.
• IgM has a u heavy chain.

IgM
• Because the pentamer has 10 antigen-binding sites,
it is the most efficient immunoglobulin in
agglutination, complement fixation (activation)
and other antibody reactions and is important in
defense against bacteria and viruses.

IgM
• It can be produced by the fetus in certain infections.
• It has the highest avidity of the immunoglobulin;
• its interaction with antigen can involve all 10 of its
binding sites.

IgD
• The immunoglobulin has no known antibody
function but may function as an antigen receptor;
• it is present on the surface of many B lymphocytes.

IgD
• It is present in small amounts in serum.
• Exists as monomer
• Mol.wt.180,000
• 0.2% of total immunoglobulin
• Cannot cross placenta

IgE
• Exists as monomer
• Mol.wt 190,000
• Four CH domains
• The Fc region of IgE binds to the surface of mast cells
and basophils.
• Bound IgE serves as a receptor for antigen (allergen).

IgE
• IgE is medically important for two reasons.
• (1) it mediates immediate (anaphylactic)
hypersensitivity
• (2) it participates in host defenses against certain
parasites e.g helminthes (worms).

IgE
• When the antigen-binding sites of adjacent IgEs are
cross-linked by allergens, several mediators are
released by the cells and immediately
(anaphylactic) hypersensitivity reactions occur.

IgE
• Although IgE is present in trace amounts in normal
serum (approx.0.004%), persons with allergic
reactivity has greatly increased amounts, and IgE
may appear in external secretions.

IgE
• IgE does not fix complement
• and does not cross the placenta.
• It binds to FcεR1 ,which is high affinity receptor
present on mast cell and basophil

IgE
• IgE is the main host defense against certain
important helminthes (worms) infections such as
strongyloides, Trichinella, Ascaris and the
hookworms, Necator and Ancylostoma.
• The serum IgE level is usually increased in these
infections.

IgE
• Because worms are too large to be ingested by
phagocytes, they are killed by eosinophils that
release worm-destroying enzymes.
• IgE specific for worm protein bind to receptors on
eosinophils, triggering the antigen-dependant
cellular cytotoxicity (ADCC) response

1-ISOTYPES
• Isotypes are antigenic determinants that
characterize classes and subclasses of heavy chains
and types and subtypes of light chains.
• Each class, subclass, type and subtype of
immunoglobulin has its unique set of isotypic
determinants.

1-ISOTYPES
• These are defined by antigenic (amino acids)
differences in their constant regions.
• Although different antigenically, all isotypes are
found in all normal humans.

1-ISOTYPES
• For example, IgG and IgM are different isotypes;
• the constant region of their H chains (γ & u) is
different antigenically ( the five immunoglobulin
classes-IgG, IgM, IgA, IgD and IgE are different
isotypes, their H chains are antigenically different).

1-ISOTYPES
• The IgG isotype is subdivided into four isotypes,
IgG1, IgG2 ,IgG3, and IgG4, based on antigenic
difference of their heavy chains.
• Similarly, IgA1 and IgA2 are different isotypes (the
antigenicity) of the constant region of their H chains
is different.) and k and λ chains are different
isotypes.

2-ALLOTYPES
• Allotypes, on the other hand, are additional
antigenic features of immunoglobulins that vary
among individuals.
• Allotypes are antigenic determinants specified by
allelic forms of the Ig genes.

2-ALLOTYPES
• They vary because the genes that code for the L and
H chains are polymorphic and individuals can have
different alleles.
• All allotypes are not found in all members of the
species.
• The prefix Allo means different in individuals of a
species

2-ALLOTYPES
• For example, the γH chain contains an allotype
called Gm, which is due to a one or two amino acid
difference that provides a different antigenicity to
the molecule.
• Each individual inherits different allelic genes that
code for one or another amino acid at the Gm site.

3-IDIOTYPES
These are the antigenic determinants formed by the
specific amino acids in the hypervariable region.
Definition - Unique antigenic determinants present
on individual antibody molecules or on molecules
of identical specificity

3-IDIOTYPES
• Identical specificity means that all antibodies
molecules have the exact same hypervariable
regions.
• Each idiotype is unique for the immunoglobulin
produced by a specific clone of antibody-producing
cells.

3-IDIOTYPES
• Anti-idiotype antibody reacts only with the
hypervariable region of the specific immunoglobulin
molecule that induced it.

IMMUNOGLOBULIN GENES

IMMUNOGLOBULIN GENES
• To produce very large number of different
immunoglobulin molecule (106-109) without
requiring excessive number of genes, special
genetic mechanism e.g DNA rearrangement and
RNA splicing, are used.
• The DNA rearrangements are performed by
recombinases.

recombination-activating genes
• Two important genes that encode recombinase are
RAG-1 and RAG-2 (recombination-activating genes).
• Mutation in these genes arrest the development of
lymphocytes and result in severe combined
immunodeficiency.

IMMUNOGLOBULIN GENES
• Each of the four immunoglobulin chains consists of
two distinct regions:
• a variable and a constant.

IMMUNOGLOBULIN GENES
• For each type of immunoglobulin chain, i.e., kappa
light chain (kL), lambda light chain (λL) and the five
heavy chains (γH, α H,µ H. εH and δH), there is a
separate pool of gene segments located on
different chromosomes.(2,22,14)

IMMUNOGLOBULIN GENES
• The genes encoding the H and L chains are
arranged in exons separated by intervening
introns.
• Each pool contains a set of different V gene
segments widely separated from the D (diversity,
seen only in H chains), J (Joining) and C (constant)
gene segment

IMMUNOGLOBULIN GENES
• In the synthesis of an H chain, for example a
particular V region is translocated to lie close to a
D segment, several J segments and a C region.

IMMUNOGLOBULIN GENES
• These genes are transcribed into mRNA and all but
one of the J segment are removed by splicing the
RNA.

IMMUNOGLOBULIN GENES
• During B cell differentiation, the first translocation
bring a VH gene near a Cu gene, leading to the
formation of IgM as the first antibody produced in
a primary response.
• Note that the joining gene does not encode the J
chain found in IgM and IgA.

IMMUNOGLOBULIN GENES
• The V region of each L chain is encoded by two
gene segments (V+J).
• The V region of each H chain is encoded by three
genes segments (V+D+J).
• These various segments are united into one
functional V gene by DNA rearrangements.

IMMUNOGLOBULIN GENES
• Each of these assembled V genes is then
transcribed with the appropriate C genes and
spliced to produce an mRNA that codes for the
complete peptide chain.

IMMUNOGLOBULIN GENES
• L and H chains are synthesized separately on
polysomes and then assembled in the cytoplasm by
means of disulfide bond to form H2L 2 units.

IMMUNOGLOBULIN GENES
• Finally, an oligosaccharide is added to the constant
region of the heavy chains and the immunoglobulin
molecule is released from the cell.

Antibody diversity
• Antibody diversity depends on
(1) multiple gene segments,
(2) their rearrangement into different sequences,
(3) the combining of different L and H chains in the
assembly of immunoglobulin molecules, and
(4) mutations.
5)A fifth mechanism called junctional diversity applies
primarily to the antibody heavy chain.

Antibody diversity
• Junctional diversity occurs by the addition of new
nucleotides at the splice junctions between the V-D
and D-J gene segments

IMMUNOGLOBULIN CLASS SWITCHING
(ISOTYPE SWITCHING)

IMMUNOGLOBULIN CLASS SWITCHING (ISOTYPE
SWITCHING)
• Initially, all B cells carry IgM specific for an antigen
and produce IgM antibody in response to exposure
to that antigen.
• Later, gene rearrangement permits the elaboration
of antibodies of the same antigenic specificity but
of different immunoglobulin classes

IMMUNOGLOBULIN CLASS SWITCHING
(ISOTYPE SWITCHING
• Antigenic specificity remains the same for the
lifetime of the B cell and plasma cell because the
specificity is determined by the variable region
genes (V,D and J genes on the heavy chains and V
and J genes on the light chain) no matter which
heavy-chain constant region is being utilized.

CLASS SWITCHING
• In class switching, the same assembled VH gene can
sequentially associate with different CH genes so
that the immunoglobulin produce later (IgG, IgA or
IgE) are specific for the same antigen as the original
IgM but have different biologic characteristics.

CLASS SWITCHING
• A different molecular mechanism is involved in the
switching from IgM to IgD.
• In this case, a single mRNA consisting of VDJ, CuCδ
is initially transcribed and is then spliced into
separate VDJ, Cu and VDJ Cδ and mRNAs.
• Mature B cells can, in this manner, express both IgM
and IgD, .

CLASS SWITCHING
• Class switching occurs only with heavy chains;
• light chains do not undergo class switching.
• Switch recombinase is the enzyme that catalyze the
rearrangement of VDJ genes during class switching.

CLASS SWITCHING
• The control of class switching is dependant on at
least two factors.
• One is the concentration of various interleukins.
• For example, IL-4 enhances the production of IgE,
whereas 1L-5 increases IgA.

CLASS SWITCHING
• If a Tfh cell recognizes the B cell’s peptide antigen,
the Tfh cell provides two key signals:
• the CD40 ligand (CD40L) molecules on the Tfh cell
bind to CD40 on the B cell,
• and the Tfh cells produce the cytokine interleukin
(IL)-21.

CLASS SWITCHING
• Together, these signals have three important effects on
the B cells:
• (1) they begin to proliferate rapidly;
• (2) they start to class switch, changing from using the
Cμ segment to using one of the other heavy chain CH
segments (Cγ, Cε, or Cα)
• and (3) they start a process of somatic hypermutation.

CLASS SWITCHING
• The other is the interaction of the CD40 protein on
the B cell with CD40 ligand protein on the helper T
Cell.
• In HYPER IgM SYNDROME, the failure to interact
properly results in an inability of the B cell to switch
to the production of IgA or IgE.
• Therefore, only IgM is made.

ALLELIC EXCLUSION
• A single B cell expresses only one L chain gene
(either κ or λ) and one H chain gene.
• In theory, a B cell could express two sets of
immunoglobulin genes, a maternal set and a
paternal set. But this is not what happens.

ALLELIC EXCLUSION
• Only one set of genes is expressed, either maternal
or paternal, and the other set is silent (i.e., it is
excluded). This is called allelic exclusion

ALLELIC EXCLUSION
• Each individual contains a mixture of B cells, some
expressing the paternal genes and others the
maternal ones.
• The mechanism of this exclusion is unknown.

CATALYTIC ANTIBODY
• Antibody can act as an enzyme to catalyze the
synthesis of ozone (O3) that has microbicidal
activity.
• Antibody can take the singlet oxygen produced by
neutrophils and react it with water to produce
hydrogen peroxide and O3.

EFFECTOR FUNCTIONS OF ANTIBODIES
• (3) stimulate immune cells through their Fc
receptors to lyse a target cell, also called antibody-
dependent cellular cytotoxicity (ADCC); and
• (4) bind and neutralize toxins and viruses.

EFFECTOR FUNCTIONS OF ANTIBODIES
• The primary function of antibodies is to protect
against infectious agents or their products .
• Antibodies provide protection because they can
• (1) activate complement to cause direct lysis of cell
membranes and promote inflammation ;
• (2) opsonize bacteria, which can occur with or
without complement;

THE PRIMARY RESPONSE
• The primary response occurs the first time that an
antigen is encountered. This usually involves T-cell–
dependent activation of B cells.
• . Most of the B cells activated from an initial
exposure undergo class switching and affinity
maturation and differentiate into plasma cells

THE PRIMARY RESPONSE
• Plasma cell produce large amounts of antibody
specific for the epitope recognized by their
immunoglobulin receptor.
• Plasma cells secrete thousands of antibody
molecules per second for a life span that lasts from
a few days to months

THE PRIMARY RESPONSE
• The first antibodies are detectable in the serum
after around 7 to 10 days in a primary response but
can be longer depending on the nature and dose of
the antigen and the route it takes to the secondary
lymphoid organ (e.g., bloodstream or draining
lymphatics).

THE PRIMARY RESPONSE
• The serum antibody concentration continues to rise for
several weeks and then declines and may drop to very
low levels.
• The first antibodies to appear in the primary response
are IgM, followed by IgG, IgE, or IgA, as more class-
switched plasma cells are generated.
• IgM levels decline earlier than IgG levels because these
plasma cells have shorter life spans

THE SECONDARY RESPONSE
• Although most activated B cells become plasma
cells, a small fraction of activated B cells become
memory B cells, which can remain quiescent in the
B-cell follicles for long periods but are capable of
being activated rapidly upon reexposure to antigen.
• Most memory B cells have surface IgG that serves
as the antigen receptor, but some have IgM

THE SECONDARY RESPONSE
• When there is a second encounter with the same
antigen or a closely related (or cross-reacting)
antigen months or years after the primary response,
the secondary response is both more rapid (the lag
period is typically only 3–5 days) and generates
higher levels of antibody than did the primary
response

THE SECONDARY RESPONSE
• The memory B cells, which underwent some degree
of affinity maturation during the primary response,
now proliferate to form a new germinal center and
repeat the process of competing for Tfh help (i.e.,
affinity maturation).

THE SECONDARY RESPONSE
• With each succeeding exposure to the antigen, the
antibodies tend to bind antigen with higher affinity
because the memory B cells are subjected to
further rounds of affinity maturation .
• During the secondary response, the amount of IgM
produced is similar to that after the first contact
with antigen

THE SECONDARY RESPONSE
• The secondary response also generates IgG plasma
cells that are greater in number and longer-lived
than those in the primary response, meaning
secondary IgG levels are higher and tend to persist
longer

THE SECONDARY RESPONSE
• This concept is medically important because the
protection afforded by the first administration of a
vaccine is less than that afforded by a booster shot.
• Furthermore, booster doses of vaccines improve
antibody binding by inducing new rounds of affinity
maturation.

RESPONSE TO MULTIPLE ANTIGENS
ADMINISTERED SIMULTANEOUSLY
• When two or more antigens are administered at the
same time, the host reacts by producing antibodies
to all of them.
• Combined immunization is widely used (e.g., the
diphtheria, tetanus, pertussis [DTP] vaccine and the
measles, mumps, rubella [MMR] vaccine)

ANTIBODIES IN THE FETUS
• In general, the fetus and the newborn have an
underdeveloped immune system that responds weakly
to infections and vaccines.
• Antibodies in the fetus are primarily IgG acquired by
transfer of maternal IgG across the placenta.
• This is why it is important to confirm the mother’s
vaccine history to ensure the newborn will be
protected.

ANTIBODIES IN THE FETUS
• Some antibodies can be made by the fetus if infection
occurs, such as in congenital syphilis.
• After birth, newborn infants can make IgG (and other
isotypes, such as IgM and IgA) to certain protein
antigens.
• For example, the vaccine against hepatitis B that
contains hepatitis B surface antigen is effective when
given to newborns

ANTIBODIES IN THE FETUS
• Most vaccines are delayed by several weeks or
months after birth to ensure the newborn’s
immune system has developed enough to respond.
After birth, maternal IgG declines, and protection
from maternal IgG is lost by 3 to 6 months.

ANTIBODIES IN THE FETUS
• After 6 months, with the loss of maternal antibody,
the risk of pyogenic infections from organisms such
as Haemophilus influenza begins to increase,
particularly in infants with B-cell deficiencies

MONOCLONAL ANTIBODIES
• Antibodies that arise in an animal in response to
typical antigens are heterogeneous, because they
are formed by several different clones of B cells
(i.e., they are polyclonal).
• Antibodies that arise from a single clone of cells are
homogeneous, or monoclonal

MONOCLONAL ANTIBODIES
• A plasma cell tumor, or multiple myeloma, is a
malignancy of a mature plasma cell clone.
• This disease can present with high levels of a
monoclonal immunoglobulin, usually IgG

MONOCLONAL ANTIBODIES
• The names of chimeric antibodies end with the suffix –
ximab, such as infliximab (anti–tumor necrosis factor [TNF])
and rituximab (anti-CD20).
• The names of humanized antibodies end with the suffix –
zumab, such as omalizumab (anti-IgE) and pembrolizumab
(anti–PD-1)
• The names of fully human antibodies end with the suffix –
umab, such as adalimumab (anti-TNF) and ipilimumab (anti–
CTLA-4)