- Antibodies, also known as immunoglobulins, are Y-shaped proteins that the immune system uses to identify and neutralize foreign objects like viruses and bacteria. They recognize and bind to a unique molecule on the pathogen called an antigen. - Antibodies are made up of two pairs of polypeptide chains called light and heavy chains that form a flexible Y shape. The variable regions at the ends of the Y determine what antigen the antibody binds to. - There are five major classes of antibodies - IgG, IgA, IgM, IgE, and IgD - that have different structures and functions like defending against pathogens in the blood or mucous membranes. IgG is the most common antibody found in

1. ANTIBODY
Mrs. Praveen Garg
VITS College, Satna

2. Introduction
• An antibody, also known as an immunoglobulin.
• It is a large, Y-shaped protein used by the immune system to identify and
neutralize foreign objects such as pathogenic bacteria and viruses.
• The antibody recognizes a unique molecule of the pathogen, called an
antigen.
• Antibodies are specialized protective proteins that bind like a lock-and-key to
the body's foreign invaders , whether they are viruses, bacteria, fungi or
parasites.
• When an antigen binds to the B-cell surface, it stimulates the B cell to divide
and mature into a group of identical cells called a clone.
• The mature B cells, called plasma cells, secrete millions of antibodies into the
bloodstream and lymphatic system.

3. Antibody structure
• Antibody is heterodimers
structrure, made up of two pair of
polypeptide chain
• The basic unit is composed of
two identical light (L) chains and
two identical heavy (H) chains,
which are held together by
disulfide bonds to form a flexible
Y shape.
• Each chain is composed of a
variable (V) region and a
constant (C) region.

4. Variable (V) and constant (C) domains within the light (L) and heavy (H) chains
of an antibody, or immunoglobulin, molecule. The folded shapes of the
domains are maintained by disulfide bonds (―S―S―).

5. Diversity in the variable domain
• Variable region contain amino terminal end and constant region contain
carboxyl terminal end.
• There are two light chain: kappa (k) and lambda chain (λ).
• Amino acid sequence of a large number of VL and VH domains revealed that
more sequence variation occur in variable region.
• These region mainly called hypervariable region. It form the antigen binding
site of the antibody molecule.
• Because the antigen binding site is complementary to the structure of
epitope, these area are now called complementarity determining regions
(CDRs).
• CDR is the antigen binding sites

6. Types of antibody
• Antibodies are grouped into five classes according to their constant region and
in activity.
• Each class is designated by a letter attached to an abbreviation of the
word immunoglobulin:
IgG(γ), IgM(μ), IgA(α), IgD(δ), and IgE(ε)
• IgG, the most common antibody, is present mostly in the blood and tissue
fluids.
• IgA is found in the mucous membranes lining the respiratory and
gastrointestinal tracts as secreted form.
• IgM found in blood and lymph fluid and are the first type of antibody made in
response to an infection.
• IgE antibodies are found mostly in the lungs, skin, and mucous membranes
and small found in blood. This can lead to an allergic reaction. They cause
mast cells to release chemicals, including histamine, into the bloodstream.
• IgD is also produced in a secreted form that is found in small amounts in blood
serum, nasal and mucous secretion.

8. IgG
• Immunoglobulin G (IgG) antibodies are large globular proteins with a
molecular weight of about 150 kDa made of four peptide chains.
• It contains two identical γ (gamma) heavy chains of about 50 kDa and two
identical light chains of about 25 kDa, thus a tetrameric quaternary structure.
• Immunoglobulin G (IgG), the most abundant type of antibody, is found in all
body fluids and protects against bacterial and viral infections.
• It represents approximately 75% of serum antibodies in humans and thus the
most common type of antibody found in the circulation.
• Each end of the fork contains an identical antigen binding site. Thus, each IgG
has two antigen binding sites.
• The heavy chain components are CH1, CH2, CH3, hinge and the VH (variable
region of heavy chain) and light chains consist of CL (Constant region of light
chain) and the κ or λ chains.
• The Fc regions of IgGs bear a highly conserved N-glycosylation site.

9. There are four IgG subclasses (IgG1, 2, 3,
and 4) in humans, named in order of their
abundance in serum (IgG1 being the most
abundant).
Subclasses IgG1, IgG2, IgG3, and IgG4 are
differentiated on the basis of the size of the
hinge region, position of interchain disulfide
bonds, and molecular weight.
The subclasses also differ in their ability to
activate complement.
IgG Subclasses

10. IgG1
• It occurs 60 to 65% of the total IgG, and responsible for the thymus-mediated
immune response against proteins and polypeptide antigens and involved in
opsonization and activation of the complement proteins. A deficiency in IgG1
isotype is a sign of a hypogammaglobulinemia.
IgG2
• It occurs 20 to 25% of the total IgG and generate immune response against
carbohydrate/polysaccharide antigens. A deficiency in IgG2 is the most
common and is associated with respiratory infections in infants.
IgG3
• IgG3 occurs 5 to 10% of total IgG and plays a major role in the immune
responses against protein or polypeptide antigens.
IgG4
• It occurs less than 4% of total IgG, IgG4 does not bind to polysaccharides.
Elevated serum levels of IgG4 are found in patients suffering from sclerosing
pancreatitis, cholangitis (inflammation in bile duct) and interstitial pneumonia
caused by infiltrating IgG4 positive plasma cells.

11. Function of IgG
• IgG is the main type of antibody found in blood and extracellular fluid and control
infection of body tissues.
• IgG is the only class of immunoglobulin that can cross the placenta in humans,
and it is largely responsible for protection of the newborn during the first months
of life.
• IgG is the major immunoglobulin in blood, lymph fluid, cerebrospinal fluid and
peritoneal fluid and a key player in the humoral immune response.
• The binding of the Fc portion of IgG to the receptor present on a phagocyte is a
critical step in the opsonization.
• IgG is produced in a delayed response to an infection and can be retained in the
body for a long time. The longevity in serum makes IgG most useful for passive
immunization by transfer of this antibody. Detection of IgG usually indicates a
prior infection or vaccination.
• Because of its relative abundance and excellent specificity toward antigens, IgG
is the principle antibody used in immunological research and clinical diagnostics.

13. IgA
• The basic monomer unit of IgA, in common with all antibodies, is arranged into
two identical Fab regions, linked through the hinge region to the Fc region, which
mediates effector mechanisms.
• Immunoglobulin A (IgA) is the first line of defence in the resistance against
infection, via inhibiting bacterial and viral adhesion to epithelial cells and by
neutralisation of bacterial toxins and virus, both extra and intracellularly.
• Immunoglobulin A (IgA), is the major class of antibody present in the mucosal
secretions of most mammals.
• It is present in mucous, tears, saliva, sweat, colostrum and secretions from
the genitourinary tract, gastrointestinal tract, prostate and respiratory epithelium.
It is also found in small amounts in blood.
• It occurs15% of the total serum immunoglobulin and thus is the second most
common human immunoglobulin in serum with serum concentration of 1 to 4
mg/ml.

15. IgA subclasses
• Two IgA subtypes exist in humans, IgA1 und IgA2.
• They differ in the molecular mass of the heavy chains and in their
concentration in serum.
IgA1
• Predominant circulating IgA1 comprising 85% of total IgA concentration in
serum.
• IgA1 shows a good immune response to protein antigens and, to a less
response against polysaccharides and lipopolysaccharides.
IgA2
• IgA2, represents only up to 15% of total IgA in serum, but IgA2 percentages
are higher in secretions.
• Plays a crucial role in the mucosa of the airways, eyes and the gastrointestinal
tract to fight against polysaccharide and lipo-polysaccaride antigens.

16. Structure of IgA
• Immunoglobulin A (IgA) is present in the serum as a 170 kDa, four polypeptide
(two L and two H) chain protein.
• Its H-chain type is alpha (α).
• IgA exists in serum in both monomeric and dimeric forms.
• Although it exists primarily in monomeric form, followed by dimeric, trimeric and
some tetrameric forms are also present.
• IgA in blood occurs in monomeric form whereas those in body secretion occur in
dimeric or multimeric forms.
• In secretions, in addition to the κ or λ L-chains and the IgA heavy chain alpha,
IgA also contains two other polypeptide chains – secretory component (SC) and
J-chain (Joining chain).
• Secretory chains help in transcytosis of exocrine IgA and stabilize IgA against
proteolytic degradation.
• The two four-chain units are held together by the J-chain through disulfide
bridges.

17. Functions of IgA
• It primarily protects mucous membranes as IgA can cross the epithelial
layer and enter into body secretion providing local immunity.
• Secretory IgA provides the primary defense mechanism against some local
infections because of its abundance in mucosal secretions (e.g., saliva and
tears).
• It prevents the passage of foreign substances into the circulatory system.
• In body secretion IgA neutralize viruses and prevent attachment on host
surface.
• Secretory IgA can also inhibit inflammatory effects of other
immunoglobulins.
•

18. IgE
• IgE exists as a monomer and is the least abundant antibody isotype in plasma,
present in a circulation and accounts for 0.002% of total immunoglobulin.
• IgE has a very short half-life which is less than 1 day. The reason is because
some proportion of the circulating amount is continually removed and destroyed
in endosomes.
• Despite the low concentrations in the circulation, IgE is extremely biologically
active. IgE antibodies bind to high-affinity receptors present on the surface of
mast cells and basophils, so that these cells may be highly sensitive to allergens
and release histamine chemicals.
• Because of this high-affinity interaction, almost all IgE produced by B cells is
bound to mast cells or basophils, which also explains the low concentration
found in circulation.
• Clinical diseases associated with unusually high elevation of serum IgE
concentrations include atopic diseases (asthma, allergic rhinitis, atopic
dermatitis, urticaria), parasitic diseases, cutaneous diseases, neoplastic
diseases, and immune deficiencies.

20. Structure of IgE
• IgE is a glycoprotein of 190KDa molecular weight produced as a monomeric
antibody comprising of 2 heavy and 2 light chains linked by disulfide bonds.
• IgE is found similar to IgG in structure, but IgE comprises two unique features:
The heavy chain which consists of high (12%) carbohydrate content and an
additional constant region (CH4).
• The structure has two identical antigen binding areas consisting of both light
chain (23KDa) and heavy chains (70KDa).
• The light chains have two domains, one constant and one variable.
• The heavy chains have five domains, one variable and four constant -region
domains.
• It is unique in having the additional constant region (CH4).
• The unique CH4 region restricts IgE binding to high-affinity receptors (Fce-RI)
on the surface of basophils and mast cells, which contain preformed granules of
heparin and histamine.
• A hinge region is absent in the structure of IgE.

21. IgM
• IgM is an antigen receptor on B cells and the first antibody produced in
an immune response.
• It is present both on B cells, and as a soluble molecule in the blood.
• Because of its large size (900 kDa), IgM is found in the intravascular space
i.e. in the bloodstream and also lymph fluid.
• Serum IgM exists as a pentamer and comprises 10% of normal human
serum Ig content.
• It is the third most abundant human immunoglobulin.
• It produce first in primary immune responses to most antigens and is the
most efficient complement-fixing immunoglobulin.

22. IgM

23. • A pentamer, where all heavy chains and all light chains are identical.
• A monomer found on B lymphocytes as B cell receptors.
• On the B cell surface this molecule is expressed on the plasma membrane as
a monomer.
• It contain an additional hydrophobic domain for anchoring in the membrane.
• In the blood, IgM is composed of pentamer unit held together by disulfide
bridges at the carboxy – terminal end of the H chains.
• Each of the five monomers within the pentamer structure is composed of two
light chains (either kappa or lambda) and two heavy chains.
• The heavy chain in IgM monomers is composed of one variable and four
constant regions.
• A hinge region is also absent in this antibody.
• J-chain is also associated with IgM in the blood and initiates the
polymerization of its subunits at the time of its secretion from a plasma cell.
Structure of IgM

24. • IgM is the first antibody produced as primary immune response.
• It is responsible for agglutination, neutralizing and cytolytic reactions and is of
vital importance in complement activation.
• IgM interacts with several other physiological molecules such as bind with
complement component C1, binds to the polyimmunoglobulin receptor (pIgR) in
a process that brings IgM to mucosal surfaces, such as the gut lumen and into
breast milk etc and have role in mucosal defence.
• IgM is the first immunoglobulin class to be synthesized by the neonate and plays
a role in the pathogenesis of some autoimmune diseases.
• Although IgM antibodies usually have low-affinity binding sites for antigen, they
have ten combining sites per molecule which can synergize with each other. It
thus has high avidity for antigens and is very efficient per molecule in dealing
with pathogens especially early in the immune response.
• Elevated levels of IgM can be a sign of recent infection or exposure to antigen.
function of IgM

25. IgD
• IgD is also produced in a secreted form that is found in very small amounts
in blood serum, representing 0.25% of immunoglobulins in serum.
• IgD has a molecular mass of 185 kDa while a half-life of 2-3 days, similar to
that of IgE.
• It is co-expressed with another cell surface antibody, IgM. B cells thus can
express both IgM and IgD which are specific for the same antigen.
• IgD starts to be expressed when the B cell exits the bone marrow to
populate peripheral lymphoid tissues. When a B cell reaches its mature
state, it co-expresses both IgM and IgD.
• IgD is also produced in a secreted form that is found in very small amounts
in blood serum.

26. Structure of IgD
• Secreted IgD is a glycoprotein
produced as a monomeric antibody
with two identical heavy chains and
two identical light chains.
• The light chains have two domains,
one variable and one constant.
• The heavy chains have four
domains, one variable and three
constant region domains.
• The IgD molecule has a long
“hinge” region (between Fab and
Fc) that appears the molecule very
susceptible to proteolytic
degradation with production of Fab
and Fc fragments and also renders
the molecule flexible, thus
enhancing antigen binding.

27. • Secreted IgD also exists and plays an exclusive function in blood, mucosal
secretions and on the surface of innate immune effector cells such as
basophils.
• IgD is also able to bind to basophils and mast cells and activate these cells
to produce antimicrobial factors that are functional in respiratory immune
defence in humans.
• It is also proposed that IgD may have some role in allergic reactions.
Function of IgD

28. Thank You