Antibodies, also known as immunoglobulins, are proteins produced by plasma cells that recognize and help eliminate antigens or microorganisms bearing those antigens. There are two main types of molecules involved in antigen recognition: antibodies and T cell antigen receptors. Antibodies exist in two forms - as membrane-bound receptors on B cells or as soluble proteins in serum and tissue fluids. The different classes of antibodies - IgG, IgA, IgM, IgD, and IgE - have distinct structures and biological functions like opsonization, complement activation, antibody-dependent cell-mediated cytotoxicity, and mediating allergic reactions. Secretory IgA plays an important role in mucosal immunity by preventing pathogen attachment and

1. Antibodies
Pramila Adhikari
CIST College
IML 261
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2. Antibodies
• Two distinct types of molecule are involved in the
recognition of antigen
– Immunoglobulins (antibodies)
– T cell antigen receptors (TCRs)
• Circulating antibodies (also called immunoglobulins) are
soluble glycoproteins that recognize and bind antigens
present in serum, tissue fluids or on cell membranes
• Their purpose is to help eliminate their specific antigens
or microorganisms bearing those antigens
• Immunoglobulins also function as membrane-bound
antigen receptors on B cells, and play key roles in B cell
differentiation
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3. Immunoglobulins
• Immunoglobulins are glycoproteins expressed as
– Membrane-bound receptors on the surface of B
cells; or
– Soluble molecules (secreted from plasma cells) and
present in serum and tissue fluids
• Contact between the B cell receptor and the antigen it
results in B cell activation and differentiation to
generate plasma cells, which secrete large amounts of
antibody
• The secreted antibody has the same binding specificity
as the original B cell receptors
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4. Immunoglobulins
• In humans, five distinct classes of immunoglobulin molecule are
recognized
– IgG – subclasses IgG1, IgG2, IgG3, IgG4
– IgA – subclasses IgA1, IgA2
– IgM
– IgD
– IgE
• They differ in size, charge, amino acid sequence, and carbohydrate
content
• The classes and subclasses together represent 9 isotypes – present
in all normal individuals
• Each immunoglobulin isotype is bifunctional (except serum IgD) –
that is they
– Recognize and bind antigen; and then
– Promote the killing and/or removal of the immune complex
formed through the activation of effector mechanism
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5. Immunoglobulins
• One part of the antibody molecule determines its
antigen specificity while another determine which
effector functions will be activated
• Effector functions include binding of the
immunoglobulin to
– Receptor expressed on host tissues (e.g. phagocytic
cells)
– The first component (C1q) of complement system
to initiate the classical pathway complement
cascade
• Different Ig classes and subclasses activate different
effector system
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7. Immunoglobulins – basic structure
• the basic structure of each immunoglobulin molecule is a unit
consisting of
– Two light polypeptide chains
– Two heavy polypeptide chains
• the amino acid sequences of the two light chains are identical ; so
are the sequence of two heavy chains
• Both light and heavy chains are folded into discrete domains, and
the type of heavy chain determines the class and subclass of the
antibody:
– μ IgM
– γ1, γ2, γ3, γ4 (IgG1, IgG2, IgG3, IgG4)
– α1, α2 IgA1, IgA2
– δ IgD
– ε IgE
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8. Antibodies are Proteins that Recognize Specific Antigens

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11. Immunoglobulins – basic structure
• The light chains (25kDa) are bound to the heavy chains
(55kDa) by interchain disulfide bridges and multiple non-
covalent interactions
• The heavy chains are similarly bound to each other by
interchain disulfide bridges and multiple non-covalent
interactions
• Each segment of approximately 110 amino acids is folded to
form a compact domain, which is stabilized through a
covalent interchain disulfide bond
– The light chain has an intrachain disulfide bond in each of
the VL and CL domains
– There is one intrachain disulfide bond in each domain of
the heavy chain
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12. Immunoglobulins – basic structure
• The characteristics fold, known as immunoglobulin
fold, is formed by the amino acid sequence of
immunoglobulin domain. This characteristics fold
defines the immunoglobulin superfamily members
• VL and VH forms antigen binding sites and CL and CH
determine effector functions
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13. Description of structure of immunoglobulin.
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•Heavy and Light chain: All chains have a four chain structure as
their basic unit.They are composed of 2 identical light chain and 2
identical heavy chain.
•Di Sulphide bonds:
•Interchain :The heavy chain and light chains and the two heavy
chains are held together by interchain disulphide bonds and by
non covalent interactions.
•Intrachain:Within each of the polypeptide chain there are also
intra chain disulphide bonds.
Variable( V) and Constant (C) region: After the amino acid
sequence of many different heavy chains and light chains were
compared,it became clear that both the heavy and light chain
could be divided into 2 regions based on variability in the amino
acid sequences:Light chain-VL(110 a.a and CL(110 a.a)
Heavy chain-VH(110 a.a and CH(330-440 a.a)

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Hinge Region: The region at which the arms of the
antibody molecule form Y is called hinge
regionn.Hinge region is absent in IgM and IgE.
Domains:3D images of the immunoglobulin
molecules show that it is not straight as depicted in
figure rather,it is folded into globular regions each of
which contains an intrachain disulphide bonds.These
regions are called domains.

15. Antibody mediated effector function
• Antibodies generally do not kill or remove pathogens
solely by binding to them. In order to be effective
against pathogens antibodies must not only recognize
antigen but also evoke responses – effector functions
– that result in removal of the antigen and death of
pathogen
• Although variable regions of the antibody are sole
agents of binding to antigen, the heavy-chain constant
region(CH) is responsible for a variety of
collaborative interations with other proteins, cells, and
tissues that result in effector function of humoral
response 15

16. Antibody mediated effector function
• Opsonization
– Protein molecules called Fc receptors (FcR), which
can bind the constant region of Ig molecules, are
present on the surfaces of macrophages and
neutrophils as well as other cells.
– The binding of phagocyte Fc receptors by several
antibody molecules with target antigen (such as
bacterial cell), produces an interaction that secures
the pathogen to phagocyte membrane
– This then results in phagocytosis of antigen-
antibody complex
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18. Antibody mediated effector function
• Antibodies activate complement
– IgM and most IgG subclasses can activate collection
of serum glycoproteins called Complement system
– Complements include group of proteins that can
perforate cell membranes
– An important byproduct of complement activation
pathway is a protein fragment called C3b, which binds
nonspecifically to cell- and antigen-antibody
complexes near the site at which complement was
activated
– The collaboration between antibody and the
complement system is important for the activation and
removal of antigens and the killing of pathogens 18

19. Antibody mediated effector function
• Antibody-dependent cell-mediated cytotoxicity
(ADCC)
– The linking of antibody bound to target cells (e.g.,
virus infected cells of host) with the Fc receptors of
a number of cell types, particularly NK cells, can
direct the cytotoxic activities of effector cell against
the target cell
– In this process, the antibody acts as newly acquired
receptor enabling the attacking cell to recognize and
kill target cell
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20. Destruction of Large Parasites by ADCC

21. Antibody mediated effector function
• Transcytosis
– Movement of Ig across epithelial layers through
mucosal surface of respiratory, GI, urogenital tracts, as
well as its export to breast milk
– In human and mice, IgA is the major antibody class that
undergoes such transcytosis
– in humans and mice most subclasses of IgG from
mother to foetus, during the third trimester of gestation
- this is a form of passive immunization, which is the
acquisition of immunity by receipt of preformed
antibodies rather than by active production of
antibodies after exposure to antigen
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22. Antibody classes and biological activities –
Immunoglobulin G
• IgG, the most abundant class in serum, constitutes about 80% of
the total serum Ig
• The IgG molecule consists of two γ heavy chains and two κ or
two λ light chains
• 4 subclasses – distinguished by differences in γ-chain constant-
region sequence and numbered according to their decreasing
serum concentrations: IgG1, IgG2, IgG3, IgG4
• IgG1, IgG3, and IgG4 readily cross placenta and play an
important role in protecting the developing fetus
• IgG3 is the most effective complement activator, followed by
IgG1; IgG2 is less efficient, and IgG4 does not activate
complement at all
• IgG1 and IgG3 bind with affinity to Fc receptors on phagocytic
cells and thus mediate opsonization
• IgG is good opsonin.
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23. IgM
• Accounts for 5% to 10%of the total serum
Ig, with an average serum concentration of
1.5 mg/ml
• Monomeric IgM is expressed as membrane-bound antibody on
B cells
• IgM is secreted by plasma cells as a pentamer in which five
monomeric units are held together by disulfide bonds that link
their carboxyl-terminal heavy-chain domains (Cμ4, Cμ4) and
their Cμ3, Cμ3 domains
• The five monomeric subunits are arranged with their Fc
regions in the centre of pentamer and the 10 antigen binding
sites on the periphery of the molecule
• Each pentamer contains an additional Fc linked polypeptide
called J (joining) chain, which is a disulfide bonded to the
carboxyl terminal cysteine residue of two of the 10μ chains. J
chain polymerizes the monomers to form pentamers
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24. IgM
• IgM is the first Ig class produced in a primary response
to an antigen, and it is also the first Ig to be synthesized
by the neonate
• An IgM molecule has high valency and can bind 10 small
hapten molecules, however due to steric hindrance, only
five or fewer molecules of larger antigens can be bound
simultaneously. Hence it is more efficient than other Ig in
binding antigens with many repeating epitopes such as
viral particles and RBC
• IgM is more efficient than IgG at activating complement
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25. IgM
• Because of its large size, IgM does not diffuse well and
therefore is found in very low concentrations in the
intercellular tissue fluids
• The presence of J chain allows IgM to bind to receptors
on secretory cells, which transport it across epithelial
linings to enter the external secretions that bathe
mucosal surfaces
• Although IgA is major isotype found in these
secretions, IgM plays an important accessory role as a
secretory Ig
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26. IgA
• Constitutes only 10-15% of total Ig in serum, it is the
predominant Ig class in external secretions such as breast
milk, saliva, tears and mucus of bronchial, genitourinary and
digestive tracts.
• In serum, IgA exists primarily as a monomer, but polymeric
forms (dimers, trimers and some tetramers) are sometimes
seen, all containing J chain polypeptide
• IgA of external secretions, called secretory IgA, consists of a
dimer or tetramer, a J-chain polypeptide, and a polypeptide
chain called secretory component which is derived from the
receptor that is responsible for transporting polymeric IgA
across cell membrane
• The J chain facilitates the polymerization of serum IgA and
secretory IgA
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27. IgA
• The secretory component is polypeptide produced
by epithelial cells of mucous membrane
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29. Formation of secretory IgA
• Secretory IgA is formed during transport through
mucous membrane epithelial cells
• Dimeric IgA binds to poly-Ig receptor on basolateral
membrane of epithelial cell and is internalized by
receptor-mediated endocytosis
• After transport of the receptor IgA complex to the
luminal surface, the poly-Ig receptor is enzymatically
cleaved, releasing the secretory component bound to
the dimeric IgA
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30. Secretory IgA
• Serves an important effector function at mucous
membrane surfaces, which are main entry sites of most
pathogenic organisms
• Because it is polymeric, secretory IgA can cross link large
antigens with multiple epitopes
• Binding of secretory IgA to bacterial and viral surface
antigen prevents attachment of pathogens to the mucosal
cells, thus inhibiting viral infection and bacterial
colonization
• Complexes of secretory IgA and antigen are easily
entrapped in mucus and then eliminated by ciliated
epithelial cells of the respiratory tract or persitalsis of gut
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31. Secretory IgA
• Secretory IgA has been shown to provide an important
line of defense against Salmonella, Vibrio Cholerae,
Neisseria gonorrhoeae and viruses such as polio,
infuenza and reovirus
• Breast milk contains secretory IgA and many other
molecules that help protect the newborn against
infection during first months of life
• Because the immune system of infants is not fully
functional, breastfeeding plays an important role in
maintaining health of newborns
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32. IgE
• Low average serum concentration (0.3μg/ml)
• IgE antibodies mediate the immediate hypersensitivity
reactions that are responsible for symptoms of hay fever,
asthma, and anaphylactic shock
• IgE binds to Fc receptors on the membrane of basophils
and tissue mast cells
• Cross-linkage of receptor bound IgE molecules by antigen
(allergen)induces basophils and mast cells to release their
granules to extracellular environment – process called
degranulation
• As a result, variety of pharmacologically active mediators
are released to cause allergic reactions and the mediators
that facilitate a build up of various cells necessary for an
ADCC antiparasitic defense
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33. IgD
• Serum concentration 30μg/ml and constitutes
about 0.2% of the total Ig in serum
• IgD together with IgM is the major membrane
bound Ig expressed by mature B cells
• No biological effector function has been identified
for IgD
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34. reference
1. Roitt, I., Brostoff, J., Male, D. Immunology. 1993. Mosby-
Year Book Europe Limited, London. 3rd Ed.
2. Kindt, T., Goldsby, R., Osborne, B. Kuby Immunology.
2007. W.H. Freeman and Company. New York. 6th edition.
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