Immunoglobulin structure and function

Immunoglobulin
Structure and Function
Onnicha Chaisetsumpan, MD
Division of Allergy, Immunology and
Rheumatology unit, Department of Pediatrics
King Chulalongkorn Memorial Hospital

Outlines
•Introduction
•Overview of B lymphocytes and humoral immune response
•Immunoglobulin structure and gene rearrangement
•Immunoglobulin function
•Therapeutic applications of immunoglobulins

Introduction
• Hallmark features of adaptive immune response: B and T lymphocyte
and their ability to recognize specific antigens
• T cells: T cell antigen receptors (TCR)
• Transmembrane molecule, recognize only antigen processed and presented in
context of MHC by antigen presenting cells
• B cells: immunoglobulin (Ig)
• Transmembrane molecule, recognized antigens in their native, properly folded
form
• Secreted molecule into plasma cells
Middleton’s Allergy: Principles and Practice, 9th edition

Introduction
Abbas Cellular and Molecular Immunology, 10th edition

Introduction
• Immunoglobulin (Ig) = antibody
• Circulating proteins produced in
vertebrates in response to exposure to
foreign structures known as antigens
• Gamma globulin: the third fastest
migrating group of globulin in
electrophoretic separation of serum or
plasma
• Immunoglobulin: immunity-conferring
portion of globulin fraction of serum or
plasma

Introduction
•Humoral immune response exhibits remarkable
diversity
• Typically generate 10 million different antibodies and
have potential to generate billions, specific for a
particular target
•Antibody function
• Recognize and bind variety of antigenic determinants
• Mediate variety of diverse, isotype-dependent biologic
effects of immunoglobulin
Monomeric antibody
molecules consists of
• Two identical heavy chains
• Two identical light chains

B lymphocytes and
Humoral Immune Response

B Lymphocytes and the Humoral Immune Response
B cells development is characterized by orderly, sequential rearrangement of
immunoglobulin heavy and light chain genes

• Transmembrane and secreted forms of
immunoglobulin from same B or plasma cell
• Same antigenic specificity
• Different at carboxyl terminus due to alternative
messenger RNA (mRNA) splicing  presence
or absence of hydrophobic transmembrane
region (tail)
• B cell differentiation into plasma cells:
preferential processing of mRNA transcripts
encoded secreted form

• Pro-B cell
• Earliest cell in B lymphocyte
lineage
• Do not produce immunoglobulin
• Expression of B lineage-restricted
surface molecules: CD9, CD10
• Pro-B cell  Pre-B cell
• RAG, TdT expression
• µ chain gene rearrangement

Pre-B cells
• Express pre-B cell receptor
• µ chain + surrogate light chain
(Vpre B + λ5 protein)
B Lymphocytes and the
Humoral Immune Response

• Pre-B cell  immature B cell
• RAG expression
• Light chain gene rearrangement
• Down regulation of surrogate
light chain
• Light chain isotype exclusion:
κ > λ
• Immature B cell = IgM
expressing cell

•Immature B cells with high-avidity self
antigen recognition
• Receptor editing or clonal deletion by
apoptosis (negative selection)
•Immature B cells with low-avidity self-
antigen recognition
• Anergy (functional unresponsive)

• Immature B cells that are not
strongly recognized self-
antigen leave bone marrow
• Maturation in spleen and
migration to other peripheral
lymphoid organs
Selected immature B cell (IgM+) 
mature B cells (IgM+ and IgD+)

Interaction of B cells with antigen
in peripheral tissue leads to
• B cell activation
• Class switching
• Somatic hypermutation
• Selection for higher affinity
antibody (affinity maturation)
• Differentiation into plasma cells
and memory B cells

B Cell Receptor Structure and Signaling
Transmembrane form of antibody
molecule associated with two signaling
chains
1. Membrane IgM and IgD, receptors
for naïve B cells
• Short cytoplasmic tails consisting of
only 3 amino acids
• Too small to transduce signals
generated after recognition of antigen
• Deliver to Igα(CD79a) and
Igβ(CD79b), containing ITAM ITAM = immunoreceptor tyrosine-based activation motif

2. BCR in class-switched B cells
• Membrane immunoglobulin: IgG, IgA or IgE
• Longer cytoplasmic tails
• Contain conserved aspartate-tyrosine-
arginine-asparagine-methionine sequence
called Ig tail tyrosine (ITT) motif
• Similar in sequence and function to CD28
cytoplasmic tail

• Antigen-induced cross-linking of membrane
immunoglobulin on B cells
• Clustering and activation of SRC family tyrosine
kinases and tyrosine phosphorylation of ITAMs in
the cytoplasmic tails of Igα and Igβ molecules
• Docking of SYK and subsequent tyrosine
phosphorylation events
• Activation of transcription factors: MYC,
NFAT, NF- κB, AP1

Role of complement and TLR on B cell activation
• Complement
• B cells express a complex of CR2 complement
receptor, CD19 and CD81
• Microbial antigens bound to complement fragment
C3d, engage both CR2 and membrane
immunoglobulin on surface of B cell
• Initiation of signaling cascades from both BCR and
CR2 complex
• Toll-like receptors
• PAMPs derived from microbes simultaneous activation
of TLR on B cells

T cell-independent VS T cell-dependent B Cell Responses
• T cell-independent antigens
• Type 1 TI antigens: bind to B cells
through PRRs (TLR), BCR
• Type 2 TI antigens: repeating
epitopes, BCRs become
extensively cross-linked and
activate B cells
• Limited low-affinity IgM
• T cell-dependent antigens
• Most prevalent
• Require T cells help for full B cell
activation

• TI and TD antigens deliver signal 1 through BCR
• Second signal results from three B and T cell
receptor-ligand interaction
1. BCR internalized antigen, process and display on B
cell MHC class II that are recognized by TCR of
antigen-specific T cells
2. B7-1 (CD80) and B7-2 (CD86) expressed on B cell
and CD28 molecule expressed on T cells
3. Induce expression of CD40 ligand (CD154) on T cells
and activation CD40 molecule on B cells
• Trigger T cell cytokine secretion
T cell-independent VS T cell-dependent B Cell Responses

TI VS TD

Immunoglobulin
Structure and Gene
Rearrangement

Immunoglobulin Protein Structure
• Purified antibody from blood
• Not possible to define antibody structure precisely
• Polyclonal antibody
• Contains a mixture of different antibodies produced by many clones of B lymphocyte
• bind to different epitopes of antigens
• Multiple myeloma, monoclonal tumor of antibody-producing plasmas cells
• Large amounts of biochemically identical antibody molecules
• Purifies to homogeneity and analyzed

• Share same basic structural characteristics
• Each B cell clone produces identical antigen-
binding sites
• Differ from other clones
• Immunoglobulin are glycoproteins
• Composed of two identical heavy chains
(HC) and two identical light chains (LC)
• Linked by disulfide bonds
• Constant region (C) and variable region (V)

• Amino-terminal variable (V) regions
• Amino acid sequences vary among
antibodies made by different B cell clones
• Antigen recognition
• Carboxy-terminal constant (C)
• Interact with other molecules and cells of
immune system
• Mediate protective or effector functions
• Antigen binding site = VH + VL

•110 amino acid residues in length
•Fold independently in a globular
motif called IgG domain
•Two layers of B-pleated sheet
•Each layer composed of 3-5 strands
of antiparallel polypeptide chain

• Regions were identified by
proteolysis of rabbit IgG molecules
• Enzyme papain acts on hinge region
• Enzyme pepsin cleaves rabbit IgG
at distal hinge region

• Amino (N)-terminal side of HC
• Fab fragments
• 2 identical fragments of intact LCs
bound to VH and CH1 regions of HC
• Bind antigen
• Fc fragments
• Bind complement or Fc receptors
(FcRs) but not to antigen
• Determined class of antibody and
effector functions
Papain cleavage  F(ab’)2 fragment

Pepsin cleavage
•Identification F(ab’)2 fragment
•Identical antigen specificity of 2 Fab regions

• Immunoglobulin: Kappa or Lambda light chains and heavy chain
• V domain (heavy chain-VDJ or light chain-VJ) fused to C domain
Human Light and Heavy-chain Immunoglobulin Gene Loci

Immunoglobulin heavy chain (H) locus: on chromosome 14
• 1 Cµ (constant) gene segment
• ~40 VH(Variable) gene segments, 25 DH (Diversity) gene segments and 6 JH
(Joining) gene segments

Light chain: Kappa or Lambda light
chains (65% Kappa)
• κ LC locus: on chromosomes 2
• 1 Ck gene segment
• ~35 Vk gene segments and 5 Jk gene
segment
• λ LC locus: on chromosomes 22
• 4 or more Cλ gene segments
• preceded by a single Jλ gene
segment and ~30 Vλ gene segments

Generation of Immunoglobulin Diversity
and Class Switch
V(D)J light- and
heavy-chain gene
rearrangement
Immunoglobulin
somatic
hypermutation
Immunoglobulin
class switch
recombination

V(D)J Recombination
• Heavy chain: two sequential
rearrangement
• First joining D to J segment
• Then V segment to the fused DJ
segment
• Light chain: single rearrangement
joins a randomly selected V gene
segment to a J segment

•Recombination signal
sequences (RSS)
• RSS-12 and RSS-23
• Lymphocyte-specific proteins
• Mediate V(D)J recombination
•Located 3’ of each V gene
segment, 5’ of each J
segment and flanking each
side of D segment
V(D)J Recombination
Modified from Immunobiology, 9th edition

• Synapsis
• Cleavage
• RAG1 and RAG2: V(D)J recombinase
• Formation of double-stranded breaks
• Hairpin opening and end processing
• Hairpins open up at coding junctions by Artemis
• Nucleotides may be added/removed
• Joining
• Brought broken coding ends together
• Ligated by double-strand break repair:
nonhomologous end joining
V(D)J Recombination

• Combinatorial diversity for V, (D) and J segments
• Junctional diversity
• Addition or subtraction of P and N nucleotides
• Combinatorial for light and heavy chains
• Combination of heavy and light chains
• Somatic hypermutation
• Antigen-driven changes in gene sequence of heavy
and light chains
Diversity of Immunoglobulin Repertoire
Combinatorial diversity for
V, (D) and J segments

V(D)J Recombination: Junctional Diversity

Human Light and Heavy-chain Immunoglobulin

Somatic Hypermutation and Class Switch
Germinal center reaction
• Germinal centers develop 4-7 days
after T-dependent B cell response
• Proliferating B cells undergoing
somatic hypermutation accumulate
in dark zone of germinal center
• Expression of enzyme activation-
induced deaminase (AID) required for
isotype switching and affinity
maturation

Somatic Hypermutation

Somatic Hypermutation
•Proliferating B cells undergo point mutations at
extreme high rate
•1 in 103 V gene base pair per cell division
•1000 times higher than spontaneous rate mutation
in other mammalian genes
•Somatic hypermutation: mutation in rearranged
V genes expressed heavy and light chains, mostly
in the antigen-binding complementarity-
determining regions (CDRs)

Class Switch Recombination
• Activated B cells in T-dependent
response undergo isotype (class)
switching and produce antibodies with
different classes of heavy chains
• Cytokines play a major role in class
switching recombination
• Switch recombination: process of
molecular mechanism of isotype
switching

I exon: initiator of transcription
Heavy chain Constant region
• Located in introns between J and C segments
• Contain numerous tandem repeats of GC-rich
DNA sequences
Switch region: DNA recombination
events involve nucleotide sequences

• Cytokine determine which CH region
will undergo germline mutation
• Germline transcription facilitates
generation of DNA-stranded breaks
• Rearrange VDJ exon recombines
with Ig heavy chain gene
Class Switch
Recombination

• AID: Activation-induced cytidine deaminase
• Key enzyme required for isotype switching and somatic
hypermutation
• Induced in activated B cells mainly by CD40 signals from Tfh cells
• Remove amino group from cytosines (C), covert C to U residues
• UNG: Uracil N glycosylase enzyme removes U residues,
leaving abasic sites
• APEI endonuclease cleaves abasic site, generating nicks at
each position
• Lead to a double-stranded break

Immunoglobulin Protein structure: Variable Regions
• Hypervariable regions
• 3 protuding loops connecting adjacent strands
of B sheets in V domains
• 10 amino acid residues long
• 3 hypervariable regions of VL and VH domain
create antigen-binding surface
• Resemble fingers protruding from each variable
domain
• Complemetanrity-determing regions (CDRs)

Immunoglobulin Protein structure: Variable Regions
• CDR1, CDR2 and CDR3
• CDR3: the most variable of CDRs
• Sequence differences among
CDRs of different immunoglobulin
molecules  distinct interaction
surfaces and specificity of
individual immunoglobulins

Immunoglobulin Protein Structure: Constant Regions
•Different isotypes and
subtypes perform different
effector functions
•Mediated by binding of
heavy chain C regions to
Fc receptors (FcRs) on
different cells

Flexibility of immunoglobulin molecules
• “Hinge region”
• Located between CH1 and CH2
• Length from 10 to more than 60 amino acid
residues in different isotypes
• Unfolded and flexible conformation  molecular
motion between CH1 and CH2 domains
• Some flexibility of immunoglobulin molecules is
due to the ability of each VH domain to rotate
with respect to the adjacent CH1 domain

Secreted VS membrane-
associated immunoglobulin
“Tail piece”
•Secreted form
• Found in blood, mucosal secretion
and other extracellular fluids
• Tail piece: carboxy-terminal
hydrophilic region

• Membrane-bound form: carboxy-terminal
stretch includes 2 segments
• Hydrophobic alpha-helical transmembrane
region
• Intracellular juxtamembrane positively
charged stretch
• Bind to negatively charged phospholipid
head groups
• On inner leaflet of plasma membrane
• IgM and IgD molecules: short, only 3 amino
acid residues in length
• IgG and IgE molecules: longer, up to 30
amino acid residues in length

Immunoglobulin Protein
Structure
•Isotype
•Allotype
•Idiotypes

•Isotype
•Antibody of different species
•Differ from others in C regions
•Recipient see antibody as foreign
•Immune response, antibody against C
regions of introduced immunoglobulin:
anti-isotypic antibody

•Allotype
• Smaller sequence differences in same species
• Inherited polymorphisms in genes encoding
C regions of heavy and light chains
• Antiallotypic antibody: antibody recognizes
allotypic determinant
•Idiotypes
• Differences between V regions concentrated in
CDRs
• Anti-idiotypic antibody: antibody recognizes
some aspect of CDRs

Antigen
• Hapten
• Small chemical
• Cannot activate B cell on their own
• Commonly attach multiple copies of small molecules
to protein or polysaccharide; carrier
• Macromolecules
• Protein, polysaccharide and nucleic acids
• Usually much bigger than antigen-binding region of
antibody molecule
• Bind to only a portion called determinant or epitope
• Antigen: any substance that may be specifically bound by an
antibody molecule or T cell receptor
• Not all antigens are capable of activating lymphocyte
• Immunogen: molecules that stimulate immune response

Immunoglobulin Structure
• Antigen binding with antibody by noncovalent,
reversible binding
• Electrostatic forces, hydrogen bonds, van der
Waals force and hydrophobic interactions
• Affinity
• Strength of the binding between a single
combining site of antibody and epitope of antigen
• Represented by dissociation constant (Kd)
• How easy to separate antigen-antibody complex
• Smaller Kd indicates stronger or higher affinity
interaction

• Avidity
• Overall strength of attachment: all
binding sites to all available epitopes
• Greater than affinity of any one antigen-
binding site
• Low-affinity IgM molecule can bind
tightly to polyvalent antigen 
produce high avidity interaction

•Immunoglobulin structures are critical for ability to recognize
antigens and their effector functions
•Features related to antigen recognition
•Ability of antibody to specifically recognize wide variety of antigen,
V regions
•Specificity, diversity and affinity maturation
•Features related to effector functions
•Mediated by Fc portions of molecules

Features related to antigen recognition
• Specificity
• Distinguish between small differences in chemical structures
• Cross-reaction: produce antibody against different but
structurally related antigen
• Diversity
• Ability of antibody to specifically bind a large number of
different antigens
• Antibody repertoire: total collection of antibodies with
different specificities
• Affinity maturation
• Subtle changes in V region structures
• Increase average binding affinity of antibodies

• Features related to effector functions
• Mediated by Fc portions of molecules
• Effector functions are initiated only by
immunoglobulin molecules that bound
antigen, not by free immunoglobulin
• Tissue distribution of antibody molecules
• Isotype switching

Immunoglobulin Function

Immunoglobulin G
• Monomer
• Major class of antibody in serum and
nonmucosal tissue
• IgG1 is the most prevalent, followed by
IgG2, IgG3 and IgG4
• Different functional properties despite
90-95% homologous
• IgG1, IgG2 and IgG3 (but not IgG4)
can activate complement cascade

Immunoglobulin G
• Unique role of IgG is providing passing
immunity to infants
• Only immunoglobulin isotype that can
cross placenta by mean of neonatal Fc
receptor (FcRn)
• IgG2 is relatively inefficient at crossing
the placenta
• Other 3 subclasses constitute most of
passively transferred maternal antibody

•Transmembrane, monomeric IgM
•Earliest immunoglobulin expressed by
developing B cells
•Antigen binding, B cell activation and
differentiation, leading to IgM secretion
•Increase in antigen-specific IgM indicates
primary immune response
Immunoglobulin M

Immunoglobulin M
•Secreted as pentamer of five IgM
monomers linked together by disulfide
bonds with J chain
•Largely found in serum
•Extremely effective at fixing
complement and neutralizing antigen
•Bind effector cells expressing Fc IgM
receptor, FcμR and Fcα/μ

• Pentameric IgM include J chain
• Can bind to poly-immunoglobulin
receptor on basolateral surface of
mucosal epithelial cells
• Facilitate immunoglobulin transport
through the epithelial barrier
• Cleave and remain bound to mucosal
immunoglobulin as the secretory
component
Immunoglobulin M

Immunoglobulin A
• Primary antibody found in mucosal immunity
• Human produced more IgA than any other class
• No reflected in serum level, major role in mucosal immunity
• IgA1 and IgA2 can exist as monomers or dimers
• IgA1 = most monomeric serum IgA
• IgA dimers contain single J chain joined to two IgA monomeric subunits
• Shorter hinge region of IgA2, more resistant to bacterial protease
• Like IgM, IgA can bind to Fcα/μ receptor
• Polymeric IgA can bind to poly-Ig receptor and be transported across
mucosal epithelium

Immunoglobulin E
• Secreted as monomer
• Play key role in response to parasitic infections
• Most IgE is bound to high-affinity IgE Fc
receptors (FcεRI) on mast dells or basophils
• Elevated in atopic persons

Immunoglobulin D
•Co-expressed with IgM on the surface of
mature B cells before antigenic stimulation
•IgM and IgD may deliver quantitatively
different signals
•IgD signaling may play an important
immune regulatory role in activation and
function of mature naïve B cells

Immunoglobulin D
•Small subset of B cells in humans
expressed IgD in the absence of IgM due
to noncanonical form of class switch
recombination that occurs in upper
respiratory mucosa
•Largely localized to upper airway, bind to
basophils and other innate immune cells
through unknown receptors
•Cross-linking promotes antimicrobial
activities

Half-life of Antibodies
• Mean time before the number of
antibody molecules is reduced by half
• Measure of how long antibodies remain
in blood after secretion from B cells or
injection in case of an administrated
antibody
• Circulating IgG molecules half-life
21-28 days

•Long half-life of IgG is attributed to
ability to bind to a specific Fc receptor
called neonatal Fc receptor (FcRn)
•Transport IgG from maternal
circulation across placental barrier
•Found on surface of endothelial cells,
macrophage
•Do not target bound IgG to lysosome
but recycles to cell surface

•Some differences in half-life
of 4 IgG isotypes
•IgG3 is relatively short-lived;
bind poorly to FcRn
•IgG1 and IgG2 are the most
long-lived and most efficient
in effector functions

Effector functions
Effector functions of antibodies are
mediated by Fc regions and triggered by
binding of antigens to the variable regions

Neutralization of microbes and microbial toxins

Antibody-mediated opsonization and phagocytosis

Leukocyte Fc Receptors
•Leukocytes express Fc receptors that bind to constant
regions of antibody
•Promote phagocytosis of immunoglobulin-coated particles
•Fc receptors for different immunoglobulin heavy chain
isotypes expressed on many leukocyte populations and serve
diverse functions in immunity

Leukocyte Fc receptors: Fcγ receptors
•Types of Fc receptors for IgG
•FcγRI, FcγRII, FcγRIII
•FcRn: neonatal Fc receptor

• Types of Fc receptors for IgG
• FcγRI, FcγRII, FcγRIII
• FcRn: neonatal Fc receptor
• FcRn
• Binding maternal IgG for transport
across placenta to fetus
• Expressed on surface of several adult
type; epithelial cells
• IgG bind to FcRn  longer serum HL

•FcγRI (CD64)
• Expressed on macrophages,
monocytes, neutrophils and
dendritic cells
• Major phagocytotic FcγR
• Only FcγR that binds
monomeric IgG1 and IgG3 with
high affinity

• FcγRII (CD32)
• 3 forms: FcγRIIA, FcγRIIB2, FcγRIIB1
• Each form recognizes same ligands
through use of same extracellular domain
• FcγRIIA: activating receptor
• FcγRIIB1 and FcγRIIB2: inhibiting
receptors
• Presence of ITIM in cytoplasmic domains
• Bind poorly to monomeric IgG but do bind
immune complex containing IgG1 and
IgG3 with higher affinity

• FcγRIII (CD16)
• 50-80 kD glycoprotein
• Attach to membrane by transmembrane tail or
linked by glycosylphosphatidylinisotol (GPI)
• Bind to IgG1, IgG3 and lectins
• Low affinity for monomeric IgG
• 2 forms: FcγRIIIA, FcγRIIIB
• FcγRIIIA: macrophage, NK cells
• FcγRIIIB: neutrophils and eosinophils
• Anchored in membrane by GPI linkage
• Lacks a signaling motif
• May trigger Ca2+ mobilization and neutrophil
degranulation

Leukocyte Fc receptors: Fcα receptors
• At least 3 distinct IgA receptors
• FcαRI (CD89)
• 55-75 kD glycoprotein
• Bind IgA1 and IgA2
• Expressed on macrophages, neutrophils and eosinophils
• FcαR on eosinophils is up regulated in atopic individuals and
likely plays key role in eosinophil activation
• IgA can also bind to Fcα/μR, an Fc receptor that also binds
to IgM
• Higher affinity for IgM than IgA
• May mediate phagocytosis of IgM- and IgA-coated bacteria

Leukocyte Fc receptors: Fcε receptors
• 2 IgE receptors: FcεRI, FcεRII
• FcεRI: high affinity IgE receptor
• Expressed by mast cells, basophils and dendritic cella and Langerhans cells in the skin
• Bind wide variety of IgE antibodies with different specificity
• Cross-linking of FcεRI on exposure to allergen  mast cell and basophil mediator
release, leading to allergic reaction
• FcεRII (CD23)
• Expressed on monocyte, B cells and dendritic cells
• Play positive and negative regulatory roles
• Buffer against the accumulation of excessive levels of IgE
• Soluble form prevent binding of IgE to FcεRI

Antibody-dependent cell-mediated
cytotoxicity (ADCC)
• NK cells receptor: FcγRIIIA (CD16) bind
to antibody-coated cells
• FcγRIII (CD16), low affinity receptor
• Bind clustered IgG molecules
displayed on cell surfaces
• Not bind circulating monomeric IgG
• ADCC occurs only when target cell is
coated with antibody molecules

Antibody-mediated clearance of helminths
• Antibody, eosinophil and mast cells function together to mediate killing and
expulsion of some helminthic parasites
• Helminths: too large to be engulfed by phagocyte, relatively resistant to microbicidal
products of neutrophils and macrophages
• Killed by toxic cationic protein called major basic protein

• 3 major pathway of complement
activation
• Classical pathway: activated by
certain isotypes of antibodies bound
to antigens
• Alternative pathway: activated on
microbial cells surfaces in absence
of antibody
• Lectin pathway: activated by
mannose-binding protein bound to
surface carbohydrates on microbes
Complement activation

• Only antibodies bound to antigens, not free
circulating antibodies, can initiate classical
pathway activation
• Each C1q molecule must bind to at least
two immunoglobulin heavy chains to be
activated
• Each immunoglobulin Fc region has only a
single C1q-binding site
Abbas Cellular and Molecular Immunology, 10th editio

• Free circulating IgM is pentameric
• Fc regions of free IgM are in inaccessible
configuration to C1q  not bind C1q
• Binding IgM to antigen induce
conformational change
• Expose C1q binding sites in Fc regions
• Efficient complement-fixing IgM>IgG
• Single molecule of IgM can bind two C1q
molecules
Abbas Cellular and Molecular Immunology, 10th editio

Functions of Complement
•Opsonization and phagocytosis
•Stimulation of inflammatory response
•Complement-mediated cytolysis

Therapeutic Applications
of Immunoglobulins

Therapeutic Applications of Immunoglobulins
Polyclonal antibody
• Immunization
• Pooled polyclonal
immunoglobulins
• Primarily IgG isolated
from healthy donors
• IVIG, SCIG
• Share antigen reactivity but typically vary in the
precise epitope recognized, affinity or even isotype
• Provide passive immunity
• Effectiveness may result from ability of IgG to bind to
Fc receptors  inhibit patient’s pathologic
antibodies from binding
• IVIG may also compete with patient’s own antibodies
for FcRn-mediated recycling of IgG  increased
rate of pathologic antibody degradation

Monoclonal antibody
• Pure collection of identical antibodies molecules with the same specificity
• Tumor of plasma cells: myeloma or plasmacytoma
• Specificity of tumor-derived antibody is not known
• Cannot be used to detect or bind molecules of interest
• Lead to idea to produce similar monoclonal antibodies of any desired specificity
by immortalizing individual antibody-secreting cells from an animal immunized
with known antigen

Monoclonal antibody
• Hybridoma: hybride of normal B cells and
myeloma tumor
• Clone with desired specificity is selected
and expanded
• Antibody specific for a single epitope on the
antigen used to immunized the animal
Therapeutic Applications
of Immunoglobulins

Monoclonal antibody applications
• Identification of phenotypic markers unique to particular cell types
• Immunodiagnosis
• Tumor identification
• Therapy: TNF, antibody against CD20
• Functional analysis of cell surface and secreted molecules

Immunoglobulin structure and function

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