Antibody, Structure, Types, Functions.

1. ANTIBODY

2. Introduction
• Antibodies are specialized glycoproteins that are formed in response
to antigen and reacts with it.
• Immunoglobulins are mainly found in vascular fluids, exocrine
secretion and on surface of lymphocytes.
• Each antibody is produce by a specific B – lymphocytes or B – cell.

3. Structure of Immunoglobulin
• The structure of immunoglobulin was given by Porter Eldeman and Nisonoff in
1959.
• The immunoglobulin are “Y” shaped structure.
• Most antibodies are bivalent.
• Have 4 polypeptide chains connected to each other by disulfide bonds.
1. Two identical Light chains have approx 220 amino acids (25kD).
2. Two identical heavy chains have approx 440 amino acids (50-70kD) and
structurally distinct from each other.
• Both chains have Constant region (CL and CH), not vary in same class and
Variable regions (VL and VH) have different sequences in different antibodies.

4. • Four chains are arranged in the form of ‘Y’ with hinge region.
• Stalk of Y is known as Crystallizable fragment (Fc) and contains site at which antibody
molecule can bind to a cell.
• The top of the Y consists of two antigen binding fragments (Fab) that bind with
compatible epitopes.
• Fc region composed of constant region and Fab fragments have both variable and
constant region.
• Heavy and light chains have homologous units (100-110 amino acids) called domain,
bounded by disulfide bond.
• Disulfide bonds also link heavy and light chains of antibody.

6. • Immunoglobulin composed of:
1. Light chains (having hypervariable
regions or complementarity
determining region or CDR)
2. Heavy chains
3. Folding domains
4. Variable region (consists of 100–
110 amino acids and differs from
one antibody to the next)
5. Constant region
6. Hyper variable region
7. Functional domains
8. Antigen binding site

8. Experiments showing antibody structure
• When the globulin fraction of serum is separated into high- and low-molecular weight
fractions, antibodies (IgG) are found in the low molecular - weight fraction (150,000-MW).
• In an experiment, digestion of IgG with the enzyme papain produced three fragments, two
of which were identical fragments (each with a MW of 45,000), were called Fab
fragments and a third was different (MW of 50,000) had no antigen binding activity but
crystallize during cold storage, so called as Fc fragment (“fragment, crystallizable”).
• Digestion with pepsin, a different proteolytic enzyme, shows antigen-binding properties of
an antibody can be separated from the rest of the molecule. Pepsin digestion generated a
single 100,000 MW fragment composed of two Fab-like fragments designated the F(ab)2
fragment, which binds antigen. The Fc fragment was not recovered because it had been
digested into multiple fragments.
• When IgG reacts with Mercaptoethanol, disulfide bonds cleaved irreversibly. If this sample
is chromatographed it shows that the intact 150,000-MW IgG molecule is composed of
subunits. Each IgG molecule contains two 50,000-MW polypeptide chains, designated as
heavy (H) chains, and two 25,000-MW chains, designated as light (L) chains.

11. Antigenic determinants on Immunoglobulins
• Isotype: Isotypic Determinants present on the constant region of
heavy- or light-chain class of antibody and used to determine the
class or sub class of serum antibody produced during an immune
response.
• Allotype: Allotypic determinants are different in amino acids
encoded by different alleles.
• Idiotype: Idiotypic determinants are arise from the variable
region of heavy and light chain. Each individual determinant of
the variable region is referred to as an idiotype.

13. Immunoglobulin classes
IgA
Occurrence- Secretion of seromucous such as saliva, nasal, fluid, sweat,
tears, genitourinary & gasrtrointestinal tracts.
Structure-
• Exist in serum in both monomeric and polymeric form consist nearly 15-
20% immunoglobulin.
• Molecular weight 150000 – 600000 D.
• It has four polypeptide chain- 2 light chain and 2 heavy chain.
• Heavy chain is two kinds based on amino acid sequences α1 and α2
depicting IgA1 and IgA2.
• Heavy chain consist of variable and constant region.
• IgA occur as dimer joined by J- chain. Dimer form is called secretory
antibody.

15. Function:
• It provide immunity against microorganism and serve as to protect excess of
antigen to immune system.
• It provide adhesion of microbes to mucosal cells.
• IgA antibodies are most abundant in the respiratory, gastro- intestinal and
genitourinary tracts where it plays a major role in protecting surface tissues from
microbial attacks.
• It binds to the viral and bacterial surface antigen preventing adherence of antigen.
• It also provides protection to the new born

16. • IgD
Occurrence- 1st observed in myeloma cell. It was found on surface of
B cell, as in traces of serum.
Structure –
• Molecular weight about – 150000 D.
• It has no interchain disulphide bonds between heavy chain
• It has 4 polypeptide chain.
• Delta as heavy chain and light chain consist of kappa or lambda.

18. Function:
• IgD is primarily B cell receptor.
• IgD play a role in antigen triggered differentiation of lymphocyte
• It function as receptor such as insulin, penicillin, is found to stimulate IgD
production.
• It has no role in neutralization of antigen.

19. IgE
Occurrence – It is found in low concentration. It is present in serum of
patients with certain types of allergies.
• These antibodies known as allergence.
Structure –
• Molecular weight is 190000 D with a short half life of 2.5 to 3 days.
• It is cytophilic antibody.
• It is attached to mast cells and basophils through its Fc portion
• It mediates for release of inflammatory molecules
• It is also known as reaginic antibody
• IgE antibody is mainly formed by plasma cells. found mostly in
secretory surface and formation influenced by T- cell.

20. Function:
• It play an important role in production of immunity to protozoa and
helminthes parasite
• It play important role in production of antibodies against
• parasite infection and role in allergic reactions
• These help protect external body surfaces from pathogenic infections.
• These are bound with mast cells which release histamines causing allergic
reactions.
• The main physiological role of IgE would appear to be the protection
of the external mucosal surface.

22. IgG
Occurrence –It is found in extra and intravascular spaces
Structure
• Molecular weight is 150000 D, sedimentation coefficient is 65-75
• Life span in a body is 18-23 days
• It has 4 subclasses IgG1,IgG2, IgG3 IgG4
• IgG antibody made up of 4 polypeptide chains-2 large heavy chains
and other 2 light chain
• At the hinges separating the base of an IgG molecule from it
branches, it can be cleaved with proteases.

23. Function:
• It activate the C1q proteins of the complement.
• IgG neutralizes the poison on toxin.
• This is the only class of antibodies that can readily cross the placenta and
plays an important role in protecting the developing foetus.
• It is bacteriolytic, viricidal and precipitating antibody.
• Its immune response appears late and persists for longer time.

25. IgM
Occurrence- Mainly found on the surface of B-cell and blood
Structure –
• Molecular weight- 900000 and sedimentation coefficient is 19
• Half life is only 10 days
• It is a large Pentamer which is composed of 4 polypeptide chain
linked by disulphide bonds
• It has 10 total antigen binding site
• It is connected by glycoprotein called J- chain which is produced
by secretary B- lymphocyte
• The 5 subunit arrange in a pin wheel array with the Fc portion in the
center

26. Function:
• It is the first Ig produced in primary response to an antigen
• It is membrane bound and function as antigen receptor on
B- cell membrane.
• It play important role in secretion of Ig due to presence of J-
chain which allows to bind receptor surface.
• These antibodies produce a fatal effect on the gram negative
bacteria which enter the blood and activate phagocytosis.
• It forms an umbrella like defence mechanism against
bacteria.

29. • Epitopes: Epitopes are the immunologically active regions on Immunogen which
binds to antigen specific membrane receptors on Lymphocytes or to secreted
antibodies. Lymphocytes may interact with a complex antigen on several levels of
antigen structure (such as Primary, Secondary, tertiary and quaternary structure of
protein).
• Receptors on Immunoglobulin Molecule: CDR regions are highly diverse in
variable region due to sequence variation. The pattern of variation is defined as:
Variabilty =
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡 𝑎𝑚𝑖𝑛𝑜 𝑎𝑐𝑖𝑑𝑠 𝑎𝑡 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛
𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑜𝑠𝑡 𝑐𝑜𝑚𝑚𝑜𝑛 𝑎𝑚𝑖𝑛𝑜 𝑎𝑐𝑖𝑠 𝑎𝑡 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛
• Complemetarity determining regions (CDRs) bind Antigen: variable loops of
beta strands, (3 on light VL & 3 on heavy chains VH) are responsible for binding
to the antigen. These loops are known as CDRs or Idiotypes.
Epitopes & Receptors on Immunoglobulins

30. References
1. Kuby, J., Goldsby, R. A, Kindt T. J., Osborne B. A. (2013). Immunology 7th edition, W.H.
Freeman and Company, New York.
2. Lyolyard, P. M., Whelan, A., Fanger. M. (2011) Instant Notes in Immunology. 3rd edition.
Garland Science Taylor and Francis Group, Newyork
3. A. K. Abbas, A. H. H.Lichtman, S. Pillai. (2017).Molecular and Cellular Immunity. 9th
edition. Elsevier
4. C. A. Janeway, P. Travers, M. Walport, M. J. Shlomchick. (2005). Immunology – the
immune system in health and Diseases. 6th edition. Garland Science Taylor and Francis
Group, Newyork
5. K. Murphy, P. Travers, M. Walport. (2008). Janeway’s Immunology. 7th edition. Garland
Science Taylor and Francis Group, Newyork
6. J. M.Cruse, R. E. Lewis. (2009). Illustrated Dictionary of Immunology. 3rd edition. CRC
Press Taylor and Francis Group, New York.
7. Google
13-Feb-21 Nidhi Saxena, Department of Microbiology 30