about antibodies, types of antibodies and structure of antibodies

1. By- Annu Maurya

2.  Antibodies are substances which are formed in the
serum and tissue fluids in response to an antigen
and react with that antigen specifically and in
some observable manner.

4.  Chemical nature of antibodies is globulin and they
are named as immunoglobulins.
 Based on sedimentation studies, most antibodies
are sedimented at 7S (M.W 150 ,000- 180,000).
Some heavier antibodies- 19S globulins (M.W
about 900 ,000) were designated as M or
macroglobulins.

6.  An antibody molecule consists of two identical
heavy and two identical light chains. The 'heavy'
(H) chains are longer and 'light' (L) chains are
shorter. Both types of chains are polypeptide in
nature. The two heavy chains are held together by
disulphide (S-S) bonds. Each light chain is also
attached to heavy chain by disulphide bond

7.  The H chains are structurally and antigenically distinct in
different classes of immunoglobulins. Five different
classes namely IgG, IgM, IgA, IgD, and IgE are
designated depending on presence of heavy chain, gamma
(y), mu (μ) , alpha (a) , delta (o) and epsilon (E)
respectively.

9.  Characteristics of five immunoglobulins present in
human sera

10.  It is the only immunoglobulin that is transported
through placenta and provides natural passive
immunity to newborn.
 Half life is about 23 days (longest amongst all the
immunoglobulins).
 It is distributed equally between the intravascular
and extravascular compartments.
 IgG appears late but persists for longer period. It
appears after the initial immune response which is
IgM in nature.

11.  Extracellular killing of target cells coated with IgG
is due to recognition of Fe receptor of IgG by killer
cells bearing the appropriat receptors.
 Four subclasses of IgG (IgGl, IgG2 , IgG3, IgG4) have
been recognised. These can be identified with specific
antisera. Differences exist in the Fe fragment of the heavy
chains of various subclasses. In human serum, these
subclasses are distributed as IgG 1 (65%), IgG2 (23%),
IgG3 (8 %) and IgG4 (4%).
 It is protective against those microorganisms which are
active in the blood and tissues.

12.  IgA is the second major seru immunoglobulin
 (about 10-13% of serum immunoglobulins). The
normal serum concentration is 0.6 - 4.2 mg/ml.
 Half life is about 6 - 8 days.
 IgA occurs in two forms , serum IgA an secretory
IgA.

13.  IgA is the principal immunoglobulin present in
secretions such as milk , saliva, tears, sweat, nasal
fluids, colostrum and in secretions of respiratory,
intestinal and genital systems. It protects the
mucous membranes against microorganisms. It
forms antibody paste on mucosa. IgA covers the
microorganisms to inhibit their adherence to
mucosal surfaces.

14.  IgA is mainly synthesised locally by plasma cells
and little is derived from serum.
 Two subclasses of IgA (IgAl and IgA2) are known.
IgA2 is predominant (60 %) in the secretions but
constitutes a minor part of serum IgA. IgA2 is
devoid of interchain disulphide bonds between H
and L chains.

15.  IgM is a pentamer consisting of 5 immunoglobulin
subunits and one molecule of J chain, which joins
the Fe region of the basic subunits.
Each H chain of IgM has four CH domains rather
than three as seen in H chain of IgG molecule.
 It constitutes about 5- 8 percent of total serum
immunoglobulins. The normal level in serum is
0.5- 2 mg/ml.
 Half life is about five days.

16.  IgM is mainly distributed intravascularly (8%).
 It is the earliest synthesised immunoglobulin by
foetus in about 20 weeks of age.
 The valency of IgM is 10 which is observed on
interactions with small haptens only. With larger
antigens, the effective valency is 5.
 Two subclasses (IgMl and IgM2) of IgM are
described. These are differentiated b characteristic
H chains i.e. μ1 and μ2 H chains.

17.  IgD resembles IgG structurally.
 IgD is present in a concentration of 3 mg per 100
ml in serum. It is mostly intravascular in
distribution.
 Molecular weight is 1,80,000 (7 S monomer) .
 Half life is about three days.
 Two subclasses (IgDl and IgD2) of IgD are known.

18.  IgE is mainly produced in the linings of
respiratory and intestinal tracts.
 Half life is 2-3 days.
 It resembles IgG in structure.
 It is heat labile

19.  High level of IgE in serum is also seen in children
with a high load of intestinal parasitism.
 It cannot cross the placental barrier or fix the
complement.
 IgE is responsible for anaphylactic type of
reaction.

20.  IgG - protects the body fluids
 IgA - protects the body surfaces
 IgM - protects the blood stream
 IgE - mediates reaginic hypersensitivity
 IgD - recognition molecule on the surface of B
lymphocytes

23.  It is a plasma cell dyscrasia in which unchecked
proliferation of one clone of plasma cells occur,
resulting in the excessive production of the
particular immunoglobulin synthesised by the
clone. Such immunoglobulins synthesised from
one clone of cells are called monoclonal. Multiple
myeloma may involve plasma cells synthesising
any of the five classes of immunoglobulins, IgG,
IgA, IgD, IgM or IgE.

24.  Abnormal heavy chains are produced in excess.
This is due to lymphoid neoplasia.

25.  It is a condition in which there is a formation of
precipitate on cooling the serum, the precipitate
redissolves on warming. This is due to presence of
cryoglobulins in blood. Cryoglobulinaemia is often
found in macroglobulinaemia, systemic lupus
erythematosus (SLE) and myelomas. Most
cryoglobulins consist of either IgG, IgM or their
mixed precipitates.

27.  Immunoglobulins are glycoproteins and can act as
immunogens when inoculated into a foreign
species.
 Differences in aminoacid sequences between
different immunoglobulin classes, subclasses and
types determine their antigeni specificity.

28.  1. lsotypes
 2. Allotypes
 3. Idiotypes

29.  These determinants are shared by all members
of the same species. On the basis of isotypic
markers on H chains, different classes of
immunoglobulins are differentiated Various H
chain markers are gamma, mu, alpha, delta and
epsilon in immunoglobulins IgG, IgM, IgA,
IgD and lgE respectively. The light chains are
also distinguished through isotypic markers
into kappa and lambda.

30. These are individual specific
determinants within a species.
Allotype markers are also present on
the constant regions of heavy and
light chains. These markers are
genetically determined.

31.  Idiotype markers are located in hypervariable
regions of the immunoglobulin molecule.
Idiotypes are specific for each antibody
molecule. By immunisation with Fab
fragments, antiidiotypic antibodies can be
produced

32.  Immunoglobulins are folded to form globular
variable and constant domains. There are four
domains in each heavy chain, one in variable
region (VH) and three in constant region (CHI,
CH2, CH3). There is one additional fourth
domain on heavy chain (CH4) in lgM and lgE
molecule. Light chain has one domain in
variable region (VL) and one in constant region
(CL) in all classes of immunoglobulin (lgG,
IgA, IgD, IgM and IgE). Each domain has a
separate function.

33.  The variable region domains (VL and VH) are
responsible for the formation of a specific
antigen binding site. The constant region
domains mediate the secondary biological
functions. The area between CH 1 and CH2
domain is hinge region where enzyme papain
acts. The CH2 region in IgG binds Clq in the
classical complement pathway, and the CH3
mediates adherence to the monocyte surface.