History:

It was observed that immune serum would
agglutinate the infectious agents, lyse bacteria, precipitate microbial agents,
neutralizer bacterial toxins. At the end of 1891, Paul Ehrlich named them
antibodies. A Tiselius and EA Kabat accomplished identification of the serum
protein fraction containing antibodies in a classical experiment in 1939. They
were able to fractionate into four: albumin, alpha, beta, gamma globulins. Electrophoretically
and chemically the antibodies are globulins and they have immunological
properties. Tiselius and Kabat showed that antibody activity was mainly
confined to gamma globulin fraction .Hence JF Herrmans appropriately named it
as immunoglobulin in 1959. WHO officially accepted this term in 1964.

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An antibody (Ab),
also known as an immunoglobulin(Ig),is a large,
Y-shaped protein produced mainly by plasma cells that
is used by the immune system to neutralize pathogens such as pathogenic bacteriaand viruses. The antibody recognizes a unique molecule of
the pathogen, called an antigen, via the Fab’s variable region

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The antigen-binding (Fab) fragment is a region on
an antibody that binds to antigens.
It is composed of one constant and one variable domain of each of the heavy and
the light chain. Each
tip of the “Y” of an antibody contains a paratope (analogous to a lock) that is specific for one
particular epitope (similarly analogous to a key)
on an antigen, allowing these two structures to bind together with precision.
Using this binding mechanism, an antibody can tag a microbe or
an infected cell for attack by other parts of the immune system, or can
neutralize its target directly.

 

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Structure of immunoglobulins

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Antibody (or immunoglobulin) molecules are glycoproteins composed of one
or more units, each containing four polypeptide chains: two identical heavy
chains (H) and two identical light chains (L).

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The amino terminal ends of the polypeptide chains show considerable
variation in amino acid composition and are referred to as the variable (V)
regions to distinguish them from the relatively constant  regions.

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 Each L chain consists of one
variable domain, VL, and one constant domain, CL. The H chains consist of a
variable domain, VH, and three constant domains CH1, CH2 and  CH3.

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 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 150,000.

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Heavy and light chains are held together by a combination of
non-covalent interactions and covalent interchain disulfide bonds, forming a
bilaterally symmetric structure.

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The V regions of H and L chains comprise the antigen-binding sites of
the immunoglobulin (Ig) molecule.

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Each Ig monomer contains two antigen-binding sites and is said to be
bivalent.

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The hinge region is the area of the H chains between the first and
second C 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.

Immunoglobulin
IgG class

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Found in highest concentration in the blood, and for this reason plays
the major role in antibody mediated defense mechanisms.

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Because this the smallest of the immunoglobulin molecules, IgG can
escape from blood vessels more easily than can the others.
(Inflammation—increase vascular permeability—IgG)

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In ruminants, especially cattle, IgG1 and not,IgA, is the major secretory
immunoglobulin in colostrum and milk.This is due to selective transfer from the
bloodstream into the mammary gland.

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Main Ig class responsible for neutralization of bacterial toxins and
antigen agglutination

Immunoglobulin
M

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Occurs in the second highest concentration after IgG in most mammalian
cells

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Major immunoglobulin produced during a primary immune response

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Also produced in secondary response but this tends to be masked by
predominance of IgG

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More efficient than IgG at complement activation, opsonization,
neutralization of virus, agglutination.

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IgG is the only class of immunoglobulins that can cross the placenta and
because of this ability IgG antibodies form the main defense barrier against
infection during the first week of life

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Main Ig class responsible for neutralization of bacterial toxins and
antigen agglutination.

Immunoglobulin
A

 

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Made in the walls of the intestine, respiratory tract, urinary system,
skin and mammary gland.

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Serum concentration in most mammals is lower than that of IgM

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In dimeric IgA, the molecules are joined by a J chain

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IgA produced in body surfaces passes through epithelial cells into
external secretions, eg-most of the IgA made in the intestinal wall is carried
into the intestinal fluid

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IgA is transported through intestinal epithelial cells bound to a
protein of 71kDa called the polymeric immunoglobulin receptor (pIgR) or
secretory component.

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Secretory component binds IgA diner to form a complex molecule called
secretory IgA (SIgA). It protect IgA from digestion by intestinal proteases.

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Secretory IgA is the major If in the external secretions of
nonruminants.

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Hence , important in protection of the intestinal, respiratory,
urogenital tracts, the mammary gland, and the eyes against microbial invasion.

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Can agglutinate particulate antigens and neutralize viruses.

 

 

Immunoglobulin
E

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Made by plasma cells located beneath body surfaces

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Typical Y shaped

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Four chains immunoglobulins with four constant domains in it’s heavy
chains

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Present in low concentration in serum

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Hence cannot act by binding and coating antigens as the other
immunoglobulins do

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Triggers acute inflammation by acting as a signal transducing molecule

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IgE binds to receptors on mast cells and basophils, when antigen binds
to IgE it triggers release of inflammatory molecules from the mast cells

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Mediates type one hypersensitivity reaction

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Immunity to parasitic worms

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Shortest half life of all immunoglobulins

Immunoglobulin
D

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Unique among the major Ig classes as it has not been detected in all
mammals

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Found in primates, rodents, cattle, sheep, pigs and probably is present
in dogs

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Not been detected in horses and rabbits and chickens

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Mainly found attached to B cells, very little of this IgD is secreted
into the blood

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