I hope this one does it

1. ANTIBODY-DEPENDENT RESPONSES
B Cell Development
• In the germ line of pluripotent stem cells,
– Constant region (C), diversity (D), joining (J) and
variable (V) region Ig genes are encoded on
chromosome14
– Heavy (H) chain gene elements are VH, D and JH
– VL and JL are for light chains of an Ig.
• Genes for Ig kappa () and lambda () are on
chromosome 2 and 22, respectively.
• Gene segments for CH arranged linearly in the
order
– Cμ, Cδ, Cγ3, Cγ1, pseudogene Cε, Cα1, Cγ2, Cγ4, Cε
and Cα2.

2. Immunoglobulins (Igs)
• Igs are members of the immunoglobulin
gene superfamily which includes
– T cell receptor; MHC class II and I molecules;
β2 – microglobulin; CD2; CD3; CD4; CD8 and
C-reactive proteins.
• Somatic recombination occurs before
contact with antigen exposure during B cell
development in the bone marrow.

3. Igs cont
• One D and one J rearrange with DJ segment and
similarly one V and J rearrange with VJ.
– V(D) J recombination is expressed in developing B
cells and recognize recombination signal sequences.
• Lymphoid progenitor cells receive signals
– Cytokines and recombinase from bone marrow stromal
cells and
– Undergo D-J joining on H-chain to become early pro-B
cells and express CD45 (B220) and class II MHC
molecules.

4. VDJ Rearrangement
• A functional VDJH rearrangement essential for
– Normal pro-B cell differentiation into pre-B
cells.
• Those that fail to make a functional VDJH
rearrangement
– Undergo apoptosis, phagocytized by bone
marrow macrophages.
• Pro-B cells become pre-B cells when they express
membrane H chains with surrogate light chains in
the pre-B cell receptor.

5. VDJ cont
• Following proliferation,
– Synthesized and µ-chain expressed on cell
membrane and
– Develops into an immature B cell that is very
sensitive to an antigen.
• Different stages in B cell development
distinguished by
– Expression of mu (µ) heavy chain, surface Ig
(SmIg) and CD antigens

6. TABLE 10: Stages in B Cell Development
Stem
cell
Early
pro-B
cell
Late
pro-B
cell
Large
pre-B
cell
Small
pre-B
cell
Immat
ure B
cell
Mature
B cell
H
chain
genes
Germ
line
D-J
joining
V-DJ
joining
VDJ
rearran
ged
VDJ
rearran
ged
VDJ
rearran
ged
VDJ
rearran
ged
L
chain
genes
Germ
line
Germ
line
Germ
line
Germ
line
V-J
joining
VJ
rearran
ged
VJ
rearran
ged
Surfac
e Ig
none none none µ chain
in pre-
B
recepto
r
µ chain
in
cytopla
sm and
on
surface
Membr
ane
IgM
Membr
ane
IgM
and
IgD

7. Clomplementarity DR(CDR)
• Each B cell generates a set of functional antibody genes to
produce
– One VDJ gene for the heavy chain and one VJ gene for
either kappa () or lambda () of light chain.
• Transcription of these genes permits formation of antibody
molecules with a single combining specificity
– Able to interact with antigenic determinant in a very
specific manner through BCR.
• Variable domains contain
– Three hypervariable (complementarity determining
regions, CDRs) comprising N-terminal domains of both
heavy and light chains.

8. Allelic Exclusion
• Mature and activated B cells may co-express surface IgM
and surface IgD.
– Express only one heavy chain allotype and one light
chain isotype, despite the genetic capacity to encode
two heavy chains and four light chains.
– A reproductive rearrangement of one allele blocks the
rearrangement of the other.
– This phenomenon is referred to as allelic or isotypic
exclusion, a cornerstone of clonal selection of B-
lymphocytes.

9. Ig Regulation cont
Regulation of B cell Development
• Interleukin – 7 (IL –7) secreted by stroma cell and
stem cell factor (SCF) initiate
– Cytoplasmic cascades resulting in expression of
proteins required for B cell development.

10. Light Chain Selection
Light chain V-J joining occurs first for kappa () chain and productive
rearrangement results into immature B cell expressing IgM () BCR
and then IgM ().
• When neither  nor  is productively rearranged, the cell undergoes
apoptosis.
• B cells usually rearrange DH and DL segments of both chromosomes
simultaneously.
• During pre-B cell stage, light chain V-J joining usually occurs first for
 (kappa) chain.
• Developing cells are positively selected when the pre-B cell receptor
binds its ligand.
• Presence of a rearranged VH or VL gene signals B cells to suppress
further recombination of genes.

11. Membrane Receptors
• Membrane Ig (SmIg) comprises IgM and IgD co-expressed
on surface of naïve virgin mature B cells.
– Antigenic specific receptors are mainly expressed on B cells
(BCR) and T cells (TCR).
• Variable domains contain
– Three hypervariable (complementarity determining region, CDR)
and constant heavy region responsible for biological functions of
the Ig.
• TCR not secreted from the T cell
– Expressed on the T cell membrane with a
– Signal transduction complex, CD3 (invariant TCR) formed from
identical subunits.

12. Ig Diversity
• The body is capable of recognizing 10 to 7 antigenic
deteminants
• Diversity is achieved through several mechanisms
including
– Expression of germ line multiple variable (V) region genes;
– VJ and VDJ combinations,
– Random assortment of H and L chains;
– Imprecise DNA recombination;
– Various mutations which may occur in the V genes of H or L
chains in the life time of a B cell and receptor editing.

13. Ig Diversity
• Antibody diversity due to a
– Combination process of immunoglobulin genes
assembled from D and J regions.
• Different V genes evolve by gene
duplication and mutation giving rise to
– 500- 1000 V-heavy chain different genes and
about 12 diversity (D) and 4 joining (J) genes.

14. Ig Diversity cont
• For both kappa (κ) and lambda (λ) glycoproteins,
– There are about 200 different V-light chain genes and 6J genes.
• Junctional diversity results from
– Addition of L imprecise joining of gene segments :
– Addition of nucleotides to the DNA sequences.
– tdT adds up to 15 nucleotides to the DNA sequence of VH and JH
regions.
• B cells produce IgM and IgD receptors simultaneously thru
alternative mRNA splicing.
– One mRNA transcribed that encodes VDJH - Cμ – Cδ.
– Ig diversity my be due to somatic hypermutation.

15. Isotype Switching
• Isotype switching increases functiona diversiy thru
stimulation and Th2 cytokine production.
– Rearranged VDJH is always expressed first with membrane Cμ in
the developing B cell with both membrane Cμ and Cδ in the
mature B cell and secreted Cμ as the plasma cell.
– B cell undergoes isotype switching from IgM to IgG.
• Sequence of appearance of immunoglobulins
– Pre-B cells appear at 13-16 weeks of gestation;
– B lymphocytes with IgM, IgG and IgD in foetal liver and 12
weeks and thereafter in peripheral blood;
– IgM first, then IgG and IgA.

16. Adult Ig Levels
• Neonate respond to antigen exposure with IgM then IgG
– Adults produce IgG 5-15 days post-infection and
– Infant elaborate only IgM for 20-30 days or up to 6
months of age.
• Adult levels are achieved in 2 yrs (IgM), 5-6 yrs (IgG) and
10 yrs (IgA).
Clonal Selection Theory
• Clonal selection theory (CST) is illustrated below
– B cells are selected by an antigen from a library or
clones of the lymphocytes.

17. Effector B cells
(Plasma cells)
Memory B ells
(Residual lgs)
Antigen
Effector T cells (CTL)
Memory T cells
Peptide from macrophages (APCs)
Fig. 12A Fig. 12B
Fig 12. Clonal Selection Theory: An antigen complementary with the specific BCR gives rise to effector plasma cells which
secrete antibodies and memory B cells. Then antigen processed and presented by macrophages is recognized by TCR on the T
cell clone which is triggered to differentiate into effector T cells (CTL) and memory T cells. Refer to the text for specific
details.

18. B Cell Proliferaion
• Each cell carries an antigenic receptor
specific for epitopes of foreign antigens.
• Complementarity
– Between the epitopes and B cell antigenic
receptors together with provision of other
signals
– Cell proliferates and differentiates into
– Antibody forming cells (AFC) or plasma cells
(effector cells) and memory cells.

19. Burnet Clonal Selection Theory
• According to Burnet’s clonal selection theory,
– Subsets of virgin immunocytes (termed clones) in
circulation acquire immunity through selection by a
specific antigen.
– Recruitment of individual B cells through their
interaction with antigenic determinants proliferate and
eventually differentiate into
• Plasma cells that secrete antibodies (effector cells) and B cells
(memory cells) capable of responding to the inducing antigen.

20. Clonal Selection cont
• Similarly in clonal selection of T cells
– Recruitment of T cells leads to differentiation
into
• Effector cytotoxic lymphocytes (CTL) that eliminate
virus, tumour cells or allografts and
• Memory T cells capable of responding to the re-
exposed antigen.

21. B Cell Activation
• Antigen specific signals are derived
– From interaction between B cells (APCs) and T cells
– Provision of cytokines (T cell dependent activation)
leading to production of specific antibodies.
• In non-specific B cell activation,
– No involvement of T cells and polyclonal antibodies are
produced.
• Natural antibodies arise independently of any
known antigenic stimulation and
– Can react with exogenous antigens.

22. Antigen Specific B Cell Actvation
• Require processing and presentation of antigen in
an immunogenic form by the accessory APCs.
– B cells appear to ingest only soluble antigens by the
process of pinocytosis.
– Antigenic determinants generated are re-expressed on
the surface of B cells in association with class II MHC
antigens.
• Leads to the activation of CD4 T cells liberating
IL-4 and IL–5 cytokines

23. Ag Specific B Cell Activation cont
• IL-4 stimulates B cell proliferation and
mediates IgM+ B cell switching to IgG and
other 1gs and
– Development of Fc- receptors on B cells.
– IL – 5 induces B cell proliferation and
differentiation into IgM secreting cells.
• A model of T cell dependent B cell
activation is illustrated below

24. T Cell Dependent B Cell Activation
Fig. 13: T cell Dependent Activation of B cells. Endocytosis of antigen
digestion and interaction with MHC molecules and CD4 T cells leads to
differentiation of B cells into plasma cells that secrete antibodies. Refer to text
for details. Source: users.rcn.com/ikimball.ma.ultranet/BiologyPages/B/B_and
T-cells.html

25. Non Specific B Cell Activation
• Polyclonal activation occurs due to
– B cell expression of membrane receptors for C3d and Epstein-Barr
virus (EBV);
– Protein A – bearing staphylococci and gram-negative bacterial
lipolysaccharides.
• Other polyclonal B cell activators include
– Mitogens eg plant lectins (phytohaemagglutinin, pokeweed
mitogen);
– Anti-Igs; antibodies to certain B cell differentiation antigens;
– Bacterial and parasite derived products; tumour promoting agents.

26. Ig Levels
• Levels of immunoglobulins produced depend on
– Ethnic background, age and sex;
– History of allergy;
– Recurrent infections or endemicity of parasites in a
particular population.
Kinetics of Primary and Secondary Responses
• Primary response characterized by
– Frst measurable immune response
– A longer lag phase and
– IgM predominant antibody.

27. Secondary IR
• Secondary response is associated with
– A second exposure to same antigen,
– Shorter lag phase,
– Quicker onset and a higher magnitude,
– Long lived memory and anamnestic
response
– IgG as the predominant antibody.

28. CD40- CD40L Interaction
• Interaction between CD40 L provide important
signals in
– B cell central functions like proliferation,
– Up regulation of membrane markers,
– Isotype switching and memory B cell formation.
• CD40 ligand (CD40L)
– Member of the tumour necrosis factor superfamily
expressed on the surface of
• Activated T cells, dendritic cells, macrophages and epithelial
cells.

29. CD40-CD40L Interaction cont
• Interaction of CD40L with CD40 on B cells leads
– B cell activation, Ig secretion and Ig class switching.
• Ig class switch involves DNA recombination in
which
– A substitution of one Ig heavy chain constant region
(CH) gene
– Effected for another one with subsequent change in the
CH region and biological activities of the Ig.

30. Kinetics of Pri and Sec IR
Fig. (Image) 14. Kinetics of Primary (10) and Secondary (20) Immune Responses: Initial administration of antigen at day O leads to a
lag phase before IgM antibodies are generated 10-14 days post-immunization period. When the antigen is again given at 28 days, as a
booster, the immune response detected is more pronounced and durable and the lag period is shorter. Primary response is characterized by
IgM antibody production while a secondary response is predominantly of IgG and other classes later Source: www.gla.ac.uk/-
jmb17n/Teaching/L2taching/memvacc/pictures/secab.jpg.

31. Immunological Tolerance
• Route of antigen administration determines nature
of response induced.
– Presentation of antigen via the gastrointestinal tract
favours tolerance.
– Intravenous injection also favours tolerance through
induction of CD8 T cell suppressor circuits.
• High zone tolerance designates the antigen dose
above immunizing level
– One below immunizing range termed low zone
tolerance.

32. Tolerance Induction
• Tolerance induced to a single epitope and no other
antigenic determinant is termed split-tolerance
– Manifests as depression of specific immune response
involving either single or all isotypes and
– Delayed type hypersensitivity or may be partial.
• Immunological tolerance established
– Early in ontogeny and in adult life partly through clonal
abortion; clonal deletion; clonal anergy or silencing.

33. Clonal Abortion
• Lymphocytes mature through a vulnerable
stage
– In which contact with a specific antigen results
into inevitable death of the clones
• When elimination involves functionally
mature lymphocytes lacking T cell help the
process is referred to as clonal deletion.

34. Clonal Anergy
• When B cells encounter autoantigens,
– There is down-regulatory signaling process
(silencing) resulting into induction of a
refractory state of hyporeactivity designated
clonal anergy.
• B cells are also rendered unresponsive through
– Antigen dependent modulation of SmIg.
– Iinteraction of antigen with IgM and IgD on
neonatal B cells leads to Ig redistribution,
capping and endocytosis of immune complexes
formed.

35. Clonal Anergy cont
• Mature B cells expressing IgM and IgD or other
isotypes
– Receptor require very high concentrations of
antigen for tolerization.
– Multivalent antigen immobilizes the antigen
receptors and freezes the membrane in
overwhelming high concentrations
– Leading to a situation referred to as
immunological paralysis

36. Basic Ig Structure
• Antibodies are glycoproteins consisting of
– Subunits containing two identical light chains (L chains) and two
identical heavy chain (H chains)
– With variable (V) regions at the N-terminal of both H and L
chains.
• Hypervariable regions in the V region
– Construct the antigen binding site designated complementarity
determining regions (CDRs)
• The C-terminus of H and L chains form
– Constant (C) regions consist of two kinds of kappa () 60% and
lamda (λ), 40% for the L chains.

37. Basic Ig Formula
• The basic formula of Ig subunit is
– H2 L2, a four chain structure (some Igs composed of
polymers of the basic monomeric form).
• Five different kinds of C regions for H chains
define isotypes (classes)
– Mu (u) chains (H chain of IgM);
– Gamma () chains (IgG);
– Alpha () chains (IgA);
– Delta () chains (IgD), and epsilon () chains (IgE).

38. Ig Structure cont
• Light chain composed of 220 amino acid residues
• Heavy chains composed of 440-550 amino acids
held together by covalent disulfide bonds.
• Variable regions
– Contained within the (NH2) terminal and of the
polypeptide chain (1-110 amino acids) and
– Constant region comprising 111-220 or 440-550 amino
acids residues.

39. Regulatory Structures on V and C
• Isotypes are constant region determinants of Ig heavy
chains
– Distinguishes various classes and subclasses of Igs.
– Determinants interact with Fc receptors, Ig binding
factors and rheumatoid factors (RF) or anti-isotype
antibodies.
• Five classes of heavy chain isotypes exist
– IgG1, IgG2, IgG3, IgG4; IgA1, IgA2; IgM; IgE and
IgD.

40. Regulatory Structures
• Idiotypes are antigenic determinants located in the
V region of various isotypes.
• Allotypes represent genetic variants (alleles)
– Minly in the constant (C) region of the IgG and
IgA and also in the V region.
– Those associated with IgG are designated Gm
– Am refer to alpha-chain IgA specific allotypes.

41. Gm and Am markers
• Immunoglobulin heavy chain (Gm) and light chain (Km)
allotypes are
– Markers of susceptibility to a wide range of immune-mediated
diseases (malignancies, infectious diseases and autoimmune
disorders)
Idiotypic network interaction
• Idiotypes are antigenic determinants located in the V
region of an Ig
– Gives rise to production of anti-idiotypic antibodies and T cells.
• Recognition structures involved are
– B cell idiotypes (V-regions of Ig) and T cell idiotypes (V-regions
of the T cell receptor).

42. TABLE 11: Immunoglobulin Profiles And Properties
Isotypes IgM Ig
D
IgG1 IgG2 IgG3 IgG4 IgA1 IgA2 sIgA IgE
Heavy chain domains
Sedimentation coefficient (S)
Carbohydrate content (%)
Molecular weight (Mw x 1000)
Serum concentrations (mgdL-1)
Classical complement activation
Maternal placental transmission
Immediate hypersensitivity
mediator
Agglutinating potency
Anti-toxin potency
Neutralizing capacity
Opsonic capacity
5
19
12
970
120
++++
-
-
+++
+
+
-
4
7-8
14
184
3
-
-
-
-
-
-
-
4
6
3
146
900
++
+
+
+
+++
+
+++
4
6
3
146
300
+
±
+
+
+++
+
+++
4
7
3
170
100
++
+
+
+
+++
+
+++
4
6
12
146
50
-
+
+
+
+++
+
+++
4
7
11
160
300
-
-
-
+
+
+
+
4
7
11
160
50
-
-
-
+
+
+
+
4
11
11
385
5
-
-
-
+
++
+++
-
5
8
12
195
0.0
5
-
-
++
+
-
-
-
-
Mean normal immunoglobulin levels in serum, colostrums, milk and saliva are ( mg /ml): IgG 6-13; IgA 0.6 – 3; IgM 05 – 3; IgD <0.14 and IgE < 0. 0004

43. Ag.Ab Reactions
• Antigens react with antibodies to form soluble or
precipitating immune complexes.
• Antibodies possess antigen-combining sites
– Formed by at least 4 regions of extreme variability in the V regions
of both H and L chains (hypervariable regions)
– Antigen makes contact with about 10 – 12 amino acids in this
region referred to as a cleft.
• When an antibody binds strongly to only the immunizing
antigen and to a lesser extent to other similar determinants
it is termed specific.
• A substance eg hapten with a single antigenic determinant
(monovalent) combines with one paratope on the antibody.

44. Law of Mass Action
• According to the Law of Mass Action,
[Ag] + [Ab] = [Ag. Ab]
(Antigen) (Antibody) (Immune complex, IC)
Equilibrium constant, K = [Ag. Ab]
[Ag] + [Ab]
• Antibody affinity refers to the strength of a single antigen-antibody
bond generated by the summation of positive and negative forces.
• Antibody avidity is the sum total of the binding forces where both the
antigen and antibody are multivalent
– Relative strength of the antigen-antibody binding is dependent on the
valency of the reacting substances.

45. Antibody Cross-Reactivity
• Some antigens display shared structural similarity
and the produced antIbody
– Possess determinants demonstrate polyspecific
reactions referred to as cross-reactivity.
• The most common heterophile substance is
– Forssman antigen found in red blood cells and
pneumococci and salmonella.
– Other shared antigens are between Treponema pallidum
bacterium and cardiolipin (mammalian heart tissue
component).

46. IgG Molecule
• IgG molecule exists as
– A monomer in membrane-bound form on the surface of B-
lymphocytes and in secreted form.
• IgG levels attained by 5-6 years of age
– Is about 150 kDa in Mw consisting of 4 classes (IgG1,
gG2, IgG3, and IgG4)
• The only Ig able to cross the placenta to protect infant.
– Selective transfer of IgG prevents maternal allergy
producing IgE antibodies and ABO isoagglutinins of
IgM accessing the child.
– Maternally acquired measles IgG interfere with active
immunization in children under 12 months of age.

47. IgG Molecule cont
• IgG is produced by plasma cells in secondary lymphoid
organs (lymph nodes and spleen)
– Constitutes about 70 – 75% of total immunoglobulins produced
daily in the body.
– Structure of IgG displays a basic structure of an Ig as depicted
below consisting of
• Two light (L) chains (Mw 25kDa) and two heavy (H) chains (Mw 55 –
77kDa) held together by inter-and intra-chain disulphide (S-S) bonds.
• Each chain is divided into
– Variable (V) antigen binding fragment (Fab) and a constant (C) or
crystallizable fragment (Fc).
• Light chains are further divided into two subtypes,
– Kappa (κ) and lamda (λ) on the basis of differences in antigenic
determinants.

48. IgG cont
• Hinge region is between the papain and pepsin cleavage
sites rich in prolines
– Separates the two combining sites
– Contribute to molecular mobility of the Ig
– Cysteines contribute to interchain disulphide bonds.
• Domains are conserved units of molecular structures of Ig
with a protein conferring a unique shape.
– Different monomers and different levels of glycosylation
contributes to variations in Ig molecular weights.
• The polypeptide chains fold into
– Three dimensional globular structures designated domains
schematically presented below

49. IgG Cleavage Sites
• Ig hydrolysis of the polypeptides at specific sites
in the hinge region results in
– IgG cleavage by pepsin digestion yields F(ab)2 and one
Fc fragment
– Papain digestion produces two Fab and one Fc
fragment as shown in Fig. 18.
• IgA molecule resistant to proteinase activity
– Hinge region made up of proline residues reinforced
by galactose – containing oligosaccharides

50. IgG Hinge Region
• Hinge region is between the papain and pepsin cleavage
sites rich in prolines
– Separates the two combining sites
– Contribute to molecular mobility of the Ig
– Cysteines contribute to interchain disulphide bonds.
• Domains are conseved units of molecular structures of Ig
with a protein conferring a unique shape.
– Different monomers and different levels of glycosylation
contributes to variations in Ig molecular weights.
• The polypeptide chains fold into
– Three dimensional globular structures designated domains
schematically presented below.

51. IgG Subclasses
• IgG subclasses consist of four polypeptides
– γ1, γ2, γ3 and γ4, designated IgG1, IgG2, IgG3
and IgG4, respectively.
• Average normal serum concentrations are
– 60 – 70% (IgG1); 14 – 20% (IgG2); 4 – 8%
(IgG3); 2 – 6% (IgG4) of normal IgG levels.

52. IgG Subclasses
• Distribution of the IgG subclasses based on
– Number and arrangement of inter-heavy chain
disulphide bonds.
– In IgG1 the disulphide bond located between
CH1 and CH2 in the constant region and
– Between VH and CH1 in IgG2, IgG3 and IgG4.

53. IgG Subclass Activities
• With respect to the number of disulphide
chain bonds
– IgG1 and IgG4 possess two, IgG2 four and
fifteen exist in IgG3.
• IgG associated biological properties differ
at the subclass level.
– Complement activation is in the order of IgG3
> IgG1 > IgG2 > IgG4.

54. IgG Subclass Activities cont
• Transmission across the placenta rates
– IgG1> IgG3 and IgG4 while IgG2
transmission is either minimal or non-existent.
• All the four IgG subclasses are potent anti-
toxin and opsonic antibodies.

55. IgM Molecule cont
IgM molecule
• IgM molecule exists as a monomer on surface of B cells
– All mature B-lymphocytes possess both IgM and IgD in the
membrane bound form as an antigenic receptor.
• Exists as a receptor pentamer with Mw of about 970 kDa
(each monomer about 180 kDa) with
– Ten antigen-binding sites.
– Monomers are joined together through disulphide bridges with J-
chain about 15 kDa.
• IgM produced in the lymph nodes and spleen (about 10%
total serum Ig produced during the primary immune
response)

56. IgM cont
• IgM rich in carbohydrates constituting
about 12% of its Mw and its heavy chain has
a 4th CH domain.
• IgM rises rapidly (3-4 weeks) of life then
gradually
– Adult levels achieved at 2 years and
intrauterine infection in infants leads to
increased IgM levels and sometimes IgA
•

57. IgM cont
• Circulating IgM molecule
– Pentamer of five four-chain polypeptides
– Held together by disulphide linkages and J-
chain polypeptides.
• Pentameric structure endows
– IgM with potent agglutinating properties and
efficient complement activation

58. IgA Molecule
Serum IgA exists as
– A monomer of about 160 kDa Mw (80%)
– Or dimmer of about 415 kDa (20%) held
together by J-chain (15 kDa).
Secretory IgA (sIgA)
• sIgA exists primarily as
– A dimer of Mw 390 kDa held together by a J-
chain (15 kDa ) and a secretory component
(polypeptide of 70 kDa in Mw)

59. sIgA cont
• A secretory component synthesized by mucosal epithelial
cells and delivered from the poly – Ig-receptor.
• IgA is then released as secretory IgA (sIgA) in the lumen
of mucosa and
– The secretory piece covalently linked to Fc of IgA and
– Wrapped around the Fc-portion of the IgA dimmer.
• The basic structure of sIgA illustrated in a planet form
below
• Combined serum and sIgA
– The most abundant Ig in the body fluids

60. Fig. 21B: A planet form of sIgA shows the secretory component (protection protein) wrapped around
the constant regions of the heavy chains. Source: www.planetbiotchnology.com/technology.htm.

61. Synthesis and Functions of sIgA
• Primed T cells regulate IgA committed B cells and the
production of IgA immunoglobulins.
• Peyer’s patches consist of
– Cuboidal epithelium cells(unique APCs called M cells, actively
pinocytic)
– Take up the antigen and deliver it to underlying mucosal lymphoid
tissue.
• T cells are sensitized and precursor IgA committed B cells
are stimulated,
– Leave the Peyer’s patches and migrate into mucosal tissue and
secretory glands.

62. sIgA cont
• B cells then differentiate into plasma cells,
– Secrete IgA specific for antigen first encountered in MALT.
• A dimeric form is produced in external secretions (saliva,
acromial, bronchial and GIT secretions)
• Secretory IgA
– Confers protection in the sub-epithelial mucosal areas(upper
respiratory, urogenital and gastrointestinal tracts)
– Present in secretions like saliva, nasal fluid, sweat and colostrum
and
– Prevents antigen uptake derived from infectious and non-infectious
agents.

63. sIgA Functions
• sIgA provides
– Protection against respiratory tract infections, allergic
reactions and autoimmune diseases.
– Immune exclusion of bacterial and viral pathogens,
bacterial toxins and noxious agents.
– Transport of pathogen immune-IgA complexes into the
bile and potentiates mucous viscosity in the blocking
and removal of infectious agents.
– Mediates antibody-dependent T cell – mediated
cytotoxicity (ADCC)
– Interferes with utilization of growth factors like iron by
bacterial pathogens.

64. sIgA Functions cont
• sIgA potentiates the activation of immune response by
– Increasing antigen uptake by M cells into Peyer’s patches and
– Lymphocyte activation through anti-idiotypic antibody stimulation.
IgE molecule
• IgE is a reagenic or homocytotropic Ig responsible for
Type I hypersensitivity reaction.
– Exists as monomer (Mw 188 kDa) in membrane-bound and
secreted form (about 0.001%, the lowest concentrated fraction of
all Igs in serum.
– Mediates ADCC through binding to Fc-R on eosinophils against
helminths.

65. IgE cont
• IgE, like IgM, has a 4th chain in the constant region.
• IgE structure similar to IgG monomer with five domains
like IgM, devoid of the hinge region.
• Monomeric IgE bound onto APC by CD23
– Leading to enhanced antigen presentation and activation of IgE -
committed B cells.
• Several mechanisms operate in the control of IgE
responses.
– IgE levels is more susceptible to both anti-idiotype and T cell
regulation.
– T cell derived IL – 4 increases IgE production while IFN – γ
decreases its production.

66. IgD Molecule
• IgD monomer about Mw 185 kDa with 14% carbohydrate
and
– Constitutes only 1% of circulating antibodies.
• The structure of IgD corresponds to a standard Ig with
– A very long hinge region of about 64 amono acid residues.
• Secreted IgD is extremely labile
– Present in very low levels in serum without any known effector
function.
• IgD appears to play an important role in the initiation of a
primary antibody response.

67. Benched Jone proteins
• Multiple myeloma has massive quantities of
one type of Ig in the serum designated
(myeloma proteins).
• Patients also secrete intact light chains in
the urine.
• The secreted light chains are termed Bence
Jones proteins.

68. Monoclonal Antibodies
• Köhler and Milstein devised a technique of
– Combining unlimited growth potential of
myeloma cells and
– Predetermined antibody specificity of normal
immune spleen cells in process referred to as
somatic cell hybridization.
• Cells immunized with a desired antigen are
fused with myeloma cells in presence of an
agent

69. Monoclonal Ab cont
• Myeloma cells lose the ability to synthesize
hypoxanthine – guanine phosphoribosyltransferase
(HGPRT) that
– Enables cells to synthesize purines and extracellular
source of hypoxanthine as a precursor.
• When cells are exposed to aminopterin (a folic
acid analog) they
– Become fully dependent on HGPRT for survival and
cannot synthesize antibodies of their own.

70. Mn Ab cont
• The cell fusion mixture is transferred to a culture
medium containing hypoxanthine, aminopterin
and pyrimidine thymidine (HAT) medium
– Unfused myeloma cells cannot grow because they lack
HGPRT and unfused normal cells have limited life
span.
– Hypbridoma cells then grown indefinitely as the spleen
cell partner supplies HGPRT and the myeloma partner
is immortal.

71. Uses of Monoclonal Abs
• Uses of monoclonal antibodies include diagnosis, research
and human therapy.
– Diagnosis fluorescent molecule used to aid in imaging the target
and a strongly – radioactive atom (Iodine-13) used to aid in killing
the target.
• In human medicine monoclonal antibodies are used in
immunosuppression of immune system such as :the
employment of muromonals – CD3 (KT3) and anti-CD3
monoclonal
– Infliximab binds to TNFD-;
– Omalizumab binds to IgE preventing it from attaching on mast
cells (allergy);
– Daclizumab binds to IL-2 receptor of activated