structure , types, functions

1. Antibody
Structure, Types and Functions
B.ASHOK KUMAR
ASSISTANT PROFESSOR
KRK GOVT DEGREE COLLEGE
ADDANKI-523201
ashokkumarzoology@gmail.com

2. Antibody
• Immunoglobulin
• 20% of total plasma proteins-Humoral immunity
• These are Glycoproteins belong to Immunoglobulin’s super
family
• They constitute most of the gamma globulins of blood
proteins.
• Rodney Porter and Gerald Edleman revealed the structure
and got noble prize in 1972.
• Large “Y” or “T” shaped protein produced by plasma cells to
neutralize pathogens
• Antibody recognises a unique antigen via Fab’s (Fragment of
Antigen Binding ) variable region
• Each tip of “Y” of an antibody contains a paratope which is
specific to an epitope of Antigen

4. Structure of Immunoglobulins
• Antibody ( immunoglobulin) - glycoproteins composed of one
or more units,
• each containing four polypeptide chains
• Two identical heavy chains (H) & two identical light chains (L).
• The amino terminal ends of the polypeptide chains show
considerable variation in amino acid composition
• Each L chain consists of one variable domain, VL, and one
constant domain, CL. (220 Aminoacids)
• The H chains consist of a variable domain, VH, and three
constant domains CH1, CH2 and CH3. (440-550 Aminoacids)
• Each heavy chain has about twice the number of amino acids
and molecular weight (~50,000) as each light chain (~25,000),
resulting in a total immunoglobulin monomer molecular
weight of approximately 150,000Dts.

6. • Heavy and light chains are held together by a
combination of non-covalent interactions and covalent
inter-chain disulfide bonds, forming a bilaterally
symmetric structure.
• The V regions of H and L chains comprise the antigen-
binding sites of the immunoglobulin (Ig) molecules.
• Each Ig monomer contains two antigen-binding sites
and is said to be bivalent.
• The hinge region is the area of the H chains between
the first and second Constant region domains and is
held together by disulfide bonds.
• This flexible hinge (found in IgG, IgA and IgD, but not
IgM or IgE) region allows the distance between the two
antigen-binding sites to vary.

8. Hyper variable regions/Hot spots
• Present in both heavy and light chains variable regions
• The Light chain variable domain include three
hypervariable regions present in between 23-34,50-56 and
89-97 amino acid moieties
• In heavy chains four hot spots present in between 1-
23,35-49, 57-88 and 98-107 amino acid moieties
• These are brought together to form Paratopes
• Paratopes are very specific to Epitopes because of these
• For Eg. IgG produced in response to different antigens
show variation in these hot spots only.

9. Hinge region
• It is an extend peptide sequence between Ch1 and CH2
domains of heavy chain
• Rich in cysteine and proline AAs , extremely variable in
amino acid sequence
• It provides flexibility to antigen binding part
• Because of proline content, hinge is highly vulnerable to
lytic enzymes viz. Papain, pepsin
• Cysteine helps in disulphide bond with opposite heavy
chain
• Hinge is absent in IgM and IgE
• Instead they have an additional constant domain which
acts like hinge

10. Disulphide bond
• Single covalent bond
• Between thiol groups of cysteine residues
• Also known as “S-S bond” or “ Disulphide bridge”
• Play an important role in the folding and stability
of some proteins secreted to EC medium
• Total number of disulphide bonds depend upon
the number of domains in the polypeptide chains
of Ig molecule.

11. Carbohydrate Moiety
• Significant amount of carbohydrate in CH2 region
• It is an oligosaccharide with variable
monosaccharides
• May include mannose as in IgM or N-acetyl
lactosamine as in IgG
• No glycosylation in Fab region
• Function is yet to know
• May help in passage through biological membranes

12. Classes of Immunoglobulins
• The five primary classes of immunoglobulins are
IgG, IgM, IgA, IgD and IgE.
• These are distinguished by the type of heavy chain
found in the molecule.
• IgGs have gamma-chains
• IgMs have mu-chains
• IgAs have alpha-chains
• IgEs have epsilon-chains and
• IgDs have delta-chains.

14. • Differences in heavy chain polypeptides allow
these immunoglobulins to function in different
types of immune responses and at particular
stages of the immune response.
• The polypeptide protein sequences responsible for
these differences are found primarily in the Fc
fragment.
• While there are five different types of heavy
chains, there are only two main types of light
chains: kappa (κ) and lambda (λ).

16. • Antibody classes differ in valency as a result of
different numbers of Y-like units (monomers) that
join to form the complete protein.
• For example, in humans, functioning IgM
antibodies have five Y-shaped units (pentamer)
containing a total of 10 light chains, 10 heavy
chains and 10 antigen-binding sites.
• IgA is a Dimer with four antigen binding sites
• IgG , IgD and IgE are monomers with two antigen
binding sites

18. IgG
• IgG, a monomer, is the predominant Ig class present in
human serum.
• Produced as part of the secondary immune response to
an antigen,
• this class of immunoglobulin constitutes approximately
75% of total serum Ig.
• IgG is the only class of Ig that can cross the placenta in
humans, and
• it is largely responsible for protection of the newborn
during the first months of life.
• Because of its relative abundance and excellent
specificity toward antigens, IgG is the principle
antibody used in immunological research and clinical
diagnostics.

19. The role of IgG in the immune response
• IgG is the major immunoglobulin in blood, lymph fluid,
cerebrospinal fluid and peritoneal fluid and
• A key player in the humoral immune response.
• Serum IgG in healthy humans presents approximately
15% of total blood proteins
• The Fc portion of IgG, but not F(ab´)2 or Fab fragments,
can cross the placenta of a mother and enter fetal
circulation, providing the fetus with postpartum
protection.
• IgG molecules are able to react with Fcγ receptors that
are present on the surface of macrophages, neutrophils
and natural killer cells, and can activate the
complement system.

20. • The binding of the Fc portion of IgG to the receptor
present on a phagocyte is a critical step in the
opsonization.
• Phagocytosis of particles coated with IgG antibodies
is a vital mechanism that cells use to cope with
microorganisms.
• 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.

21. IgG subclasses
• There are four IgG subclasses
• The subclasses differ in the number of disulfide bonds and the
length and flexibility of the hinge region.
• Except for their variable regions, all immunoglobulins within one
class share about 90% homology, but only 60% among classes.
• Determination of IgG subclasses can be a valuable tool in
indicating a potential antibody deficiency.
• Selective IgG subclass deficiencies are associated with disease.
• In cases with prolonged or severe infections, determination of IgG
levels can provide additional insight into the manifestation of
disease.
• It is important to interpret IgG subclass concentrations in
correlation to the donor's age since the immune system matures
during childhood.
• Because of its relative abundance and excellent specificity toward
antigens, IgG is the principle antibody used in immunological
research and clinical diagnostics.

22. IgG class
• Properties of IgG:
• Molecular weight: 150,000
• H-chain type (MW): gamma (53,000)
• Serum concentration: 10 to 16 mg/mL
• Percent of total immunoglobulin: 75%
• Glycosylation (by weight): 3%
• Distribution: intra- and extravascular
• Function: secondary response

23. IgG1
• IgG1 comprises 60 to 65% of the total IgG
• Predominantly responsible for the thymus-
mediated immune response against proteins and
polypeptide antigens.
• IgG1 binds to phagocytic cells and can activate the
complement cascade.
• IgG1 immune response can already be measured in
newborns and reaches its typical concentration in
infancy.
• A deficiency in IgG1 isotype is typically a sign of a
hypogammaglobulinemia.

24. IgG2
• IgG2, the second largest of IgG isotypes,
• Comprises 20 to 25% of the main subclass
• The prevalent immune response against
carbohydrate/polysaccharide antigens.
• “Adult” concentrations are usually reached by 6 or
7 years old.
• Among all IgG isotype deficiencies, a deficiency in
IgG2 is the most common and is associated with
recurring airway/respiratory infections in infants.

25. IgG3
• IgG3 comprises around 5 to 10% of total IgG
• plays a major role in the immune responses
against protein or polypeptide antigens.
• The affinity of IgG3 can be higher than that of
IgG1.

26. IgG4
• Comprising usually less than 4% of total IgG
• IgG4 does not bind to polysaccharides.
• In the past, testing for IgG4 has been associated
with food allergies, and
• Elevated serum levels of IgG4 are found in
patients suffering from sclerosing pancreatitis,
cholangitis and interstitial pneumonia caused by
infiltrating IgG4 positive plasma cells.
• The precise role of IgG4 is still mostly unknown.

27. IgM class
• Properties of IgM:
• Pentamer , Largest antibody
• Molecular weight: 900,000
• H-chain type (MW): mu (65,000)
• Serum concentration: 0.5 to 2 mg/mL
• Percent of total immunoglobulin: 10%
• Glycosylation (by weight): 12%
• Distribution: mostly intravascular
• Function: primary response
• Eliminates pathogens in early humoral immunity
before sufficient IgG.

28. IgA class
• Properties of IgA:
• Dimer , Secretory antibody
• Molecular weight: (Alpha)320,000 (secretory)
• H-chain type (MW): alpha (55,000)
• Serum concentration: 1 to 4 mg/mL
• Percent of total immunoglobulin: 15%
• Glycosylation (by weight): 10%
• Distribution: intravascular and secretions like saliva,
tears, breast milk, mucosal area of gut, Respiratory
tract, Urinogenital tract
• Function: protect mucus membranes and prevents
pathogen colonization

29. IgD class
• Properties of IgD:
• Monomer
• Molecular weight: 185,000Dts
• Half life of 2-3 days
• H-chain type (MW): delta (70,000)
• Serum concentration: 0 to 0.4 mg/mL
• Percent of total immunoglobulin: 0.25%
• Glycosylation (by weight): 13%
• Distribution: lymphocyte surface
• Function: 1.an antigen receptor on B-cells which are not
exposed to antigens
• 2.it activates basophils and mast cells to produce
antimicrobial factors

30. IgE class
• Properties of IgE:
• Discovered by Teruka and Kimishige Ishizaka in 1966
• Scarce isotype found only in mammals
• Molecular weight: 190,000Dts
• Half life of 2-3 days
• H-chain type (MW): epsilon (73,000)
• Serum concentration: 10 to 400 ng/mL
• Percent of total immunoglobulin: 0.002%
• Glycosylation (by weight): 12%
• Distribution: basophils and mast cells in saliva and
nasal secretions
• Function: protect against parasites and allergy.

31. Functions of Antibodies
• Neutralization
• Agglutination
• Precipitation
• Opsonization
• Complement activation(Fixation)
• Activation of Effector Cells
• Natural Antibodies

32. Neutralization
• Neutralizing antibodies block parts of the
surface of bacterial cell or virion to render its
attack ineffective

33. Agglutination
• “Glue together” foreign cells into clumps
• Clumps are attractive targets for phagocytosis

34. Precipitation
• Glue-together serum soluble antigens
• Forcing them to precipitate out of solution in
clumps
• Clumps are attractive targets of phagocytosis

35. Complement activation
• Fixation in which antibodies latched to foreign
cell
• Encourage complement to attack it with
membrane attack complex which leads to
– Lysis of the foreign cell
– Encouragement of inflammation by
chemotactically attracting inflammatory cells

36. Activation of Effector cells
• By coating the pathogen antibodies can
activate effector functions like
– Phagocytosis
– Mast cells and neutrophils to degranulate
– Natural killer cells will release cytokines and
cytotoxic molecules
– Ultimately results in destruction of invading
microbe

37. Natural antibodies
• Humans and higher primates
• Defined as “antibodies produced without any
previous infection, vaccination, other foreign
exposure or passive immunization”
• Can activate classical complement pathway leads
to lysis of enveloped virus particles prior to
adaptive immunity
• Rejection of Xenotransplanted organs is the result
of Natural antibodies