Antibodies, also known as immunoglobulins, are Y-shaped glycoproteins produced by plasma cells that recognize and bind to specific antigens. They have two antigen binding fragments and a flexible stem region that allows them to bind antigens separated by varying distances. There are five main classes of antibodies - IgG, IgA, IgM, IgD, and IgE - that have different structures and functions such as activating the complement system, opsonization, mucosal immunity, hypersensitivity reactions, and B cell activation. Antibodies bind antigens with high specificity and affinity through variable regions in their light and heavy chains that recognize a diverse range of molecular shapes.

1. ANTIBODIES

2. DEFINITION:
 An antibody or immunoglobulin (Ig) is a
glycoprotein that is made by plasma cells in
response to an antigen and can recognize and bind
to the antigen that caused its production.
 Antibodies bind antigen with a high degree of
specificity and affinity. Antibodies recognize a
variety of three-dimensional shapes (amino acids,
lipids, carbohydrates, etc.).
 Antibodies have more than one antigen combining
site Some bivalent Ab molecules can combine to
form multimeric Abs that have upto 10 combining
sites

3. BASIC STRUCTURES:
 An antibody consists of four polypeptides: two identical
light chains and two identical heavy chains form a Y-
shaped molecule.
 Each light chain is connected to a heavy chain by a
disulfide bond and the two heavy chains are connected
by two disulfide bonds.
 Antigen binding and effector domains are separated by a
hinge region. The hinge region allows the two antigen
binding domains to move, enabling them to bind antigens
that are separated by varying distances.

4.  Both light and heavy chains contain 2 different
regions
i. constant and
ii. variable region
 The constant (C) region is conserved among clones
and is required for structural integrity and effector
functions
 The variable (V) region varies between clones and is
involved in antigen recognition.
 The four chains are arranged in the form of a
flexible “Y” with the hinge region and is termed as
crystallizable fragment (Fc) and contains the site at
which Ab binds.
 Top of the “Y” consist of two Ag binding fragments
(Fab) that bind with antigenic determinant sites.

5. LIGHT CHAIN:
 The light chain may be either of two distinct forms called
“Kappa” and “Lambda” and can be distinguished by a.a.
sequence of carboxyl portion of the chain. Each Light chain
contains one V domain and one C domain.
 Each light chain consist of 220 a.a. and has a mass of approx.
25kDa.
HEAVY CHAIN:
 In the heavy chain NH2 terminal has a pattern of variability
similar to that of kappa and lambda of the light chain. Each
Heavy chain contains one V domain and at least three C
domains.
 Each heavy chain consists of about 440 aa and has a mass of
50-70kDa.

6. BASIC STRUCTURE OF IMMUNOGLOBULIN

9. BINDING OF ANTIGEN BY ANTIBODY - TERMS AND CONCEPTS
Antibodies bind antigens by reversible
non-covalent interactions.
Epitope: The parts of an antigen
recognized by an antibody are called
epitopes. Epitopes can be recognized on
the basis of sequence or shape.
Epitope
Epitope
Affinity: The strength with which one
antigen-binding surface of an antibody
binds an antigen.
Paratops: is a part of an antibody which
recognizes and binds to an antigen
Avidity: Each Isotype has between 2 and 10 antigen binding sites. Therefore, each
antibody can bind 2 to 10 epitopes of an antigen, as long as identical epitopes are
sufficiently close together, e.g. microbial cell surface proteins. In this case the
binding is much greater than the affinity of a single antibody-antigen bond and
is called the avidity.

10. FEATURES OF ANITBODY-
MEDIATED
ANTIGEN RECOGNITION
4. Signaling triggers B lymphocyte activation.
3. Antigen recognition is mediated by specific
domains of the antibody.
2. Each clone is specific for a single antigen.
1. Antibodies recognize a large array of
3-D structures

11. IMMUNOGLOBULINS VARIANTS:

12. Immunoglobulin Function
 Each end of the immunoglobulin molecule has a
different role.
 The Fab region is concerned with binding to antigen,
whereas the Fc region mediates binding to host tissue,
various cells of the immune system, some phagocytic
cells, or the first component of the complement system.
 The binding of an antibody with an antigen usually does
not cause destruction of the antigen or of the
microorganism , cell, or agent to which it is attached.
 Rather the antibody serves to mark and identify the target
for immunologic attack and to activate nonspecific
immune responses that can destroy the target.

13. For example:-
 Bacteria that are covered with antibodies are
better targets for phagocytosis by neutrophils
and macrophages.
 The alteration of the surface of bacteria, viruses
and other particles so that they can be more
readily phagocytized is termed opsonization.
 Immune destruction also is promoted by
antibody-induced activation of the classical
pathway.

14. IMMUNOGLOBULIN CLASSES:

15. IMMUNOGLOBULIN CLASSES:
1. IgG:-

17. FUNCTIONS:
 Immunity to new born.
 Potent activator of complement system.
 Neutralisation of toxins.
 IgG3 binds to Fc receptors- phagocytosis.

18. 2. IgM:

19. FUNCTIONS:
 Activation of classical pathway.
 Defence against multivalent bacterial antigens.
 Acts as opsonin.
 Present on B-cell surface, acts as antigen
receptor.
 IgM is the first antibody produced by naive B
cells. It has very high avidity (10 antigen binding
sites) and is involved in complement activation.
 IgM exists as pentamers

20. 3. IgA:-

21. FUNCTIONS:
 Secretory antibody.
 Effective against bacterial like
Salmonella, Vibrio cholera ,
Niesseria gonorrhoea, etc.
 Effective against viruses like those causing
Poliomyelitis & Influenza.
 Protection to infant gut.
 Mucosal immunity, neonatal passive immunity

22. 4.IgD:

23. FUNCTIONS:
 B cell activation .
 Acts as receptor for Ag binding.
 Function unknown. Restricted to membrane and not
expressed on active B lymphocytes. IgD knock-out mice
do not have any apparent defects.

24. 5.IgE:

25. MAST
CELL
Fc
Receptor
IgE
AntigenFUNCTIONS:
 Binds to Fc receptors on
basophils and mast cells.
 Responsible for immediate
hypersensitivity or allergic
reactions.
 Release of substances like
histamine , bradykinin and
other vasoactive ‘mediators’.
 IgE is involved in mast cell
activation. IgE binds mast
cells (and eosinophiles)
where it waits for antigen.
This is different from IgG,
which must bind antigen
before cells. When an
antigen cross-links IgE, it
causes mast cell
degranulation.

26. PROPERTIES OF IMMUNOGLOBULINS:
IgG IgA IgM IgD IgE
1. Serum conc.
(%)
85 5-15 5-10 <1 <1
2. Mol. Wt. 160,000 170,000 &
385,000
960,000 184,000 188,105
3.Sed. coeff. 7S 7S 19S 7S 8S
4.Heavy chain
class
Gamma Alpha Mu Delta Epsilon
5.Light chain K & L K & L K & L K & L K & L
6. Valency 2 2 or multiple
of 2
5 (10) 2 2
7.No of basic 4-
polypeptide
chains
Monomer Dimer or
Trimer
Pentamer Monomer Monomer

27. IgG IgA IgM IgD IgE
8.Placental transport + _ _ _ _
9.Present in milk + + _ _ _
10.Selectie secretion by
seromucus gland
_ + _ _ _
11. Intravascular
distribution(%)
45 42 80 75 50
12.Carbohydrate (%) 3 11 10 13 12
8.Subclasses IgG1-4 IgA1-2 _ _ _

28. IgG IgM IgA IgD IgE
14.Comple-
ment
fixation
A.Classical ++ _ +++ _ _
B.Alternati
ve
_ + _ _ _
15.Half life
(days)
23 6 5 2-3 2-3
16.Princip-
al site of
action
Serum Secretion Serum Receptor
for B cells
Mast cells
17.charact-
eristic
propert-
ies
precipitins,
antitoxins,
compleme-
nt fixation,
late Ab
Serum and
secretory
Abs
Agglutinin,
opsonin ,
lysin , early
Ab
Not known
(B-cell
activation)
Reaginic
Ab (anaph-
ylaxis)