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Adaptive Immunity:
The ability of the body to defend itself against specific invading agents such as
bacteria, toxins, viruses, and foreign tissues is called adaptive (specific)
immunity
...

Two properties that distinguish adaptive immunity from innate immunity:
(1) specificity for particular foreign molecules (antigens), which also involves
distinguishing self from nonself molecules, and
(2) memory for most previously encountered antigens so that a second encounter
prompts an even more rapid and vigorous response
...

Immunogenicity is the ability to provoke an immune response by stimulating
the production of specific antibodies, the proliferation of specific T cells, or
both
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 Reactivity is the ability of the antigen to react specifically with the antibodies or
cells it provoked
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 Chemical components of bacterial structures such as flagella, capsules, and cell
walls are antigenic, as are bacterial toxins
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 Typically, certain small parts of a large antigen molecule act as the triggers for
immune responses
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 Most antigens have many epitopes, each of which induces production of a
specific antibody or activates a specific T cell
...
Most often, they are proteins
...

 Complete antigens usually have large molecular weights of 10,000 daltons or
more, but large molecules that have simple, repeating subunits—for
example, cellulose and most plastics— are not usually antigenic
...

 A hapten can stimulate an immune response only if it is attached to a larger
carrier molecule
...

 Likewise, some drugs, such as penicillin, may combine with proteins in the
body to form immunogenic complexes
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The carrier molecule is a
non-antigenic
component and helps in
provoking the immune
response
...

Low Molecular Weight (Less than 10,000)
Haptens can react specifically with its corresponding antibody
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Diversity of Antigen Receptors
 An amazing feature of the human immune system is its ability to
recognize and bind to at least a billion (109) different epitopes
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 Cells of the immune system can even recognize artificially made
molecules that do not exist in nature
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Immunoglobulins
Antibodies or Immunoglobulins are globular proteins present in the serum and
tissue fluids
...

Antibodies play a very crucial role in the immune system of an organism
...

This enables our immune system to detect foreign organisms such as invading
pathogens, or its products and initiate the mechanism to eliminate these foreign
particles
...


The immunoglobulins constitute about 20 – 25% of the total serum proteins
...
These immunoglobulins variants are called
as Isotypes
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These peptide chains are named as two
identical Heavy Chains and two identical
Light Chains
...

The light chains are smaller and lighter in
weight with a molecular weight of 25 kDa
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Both H chains and L chains are connected through disulphide
Some antibodies are very complex as in Immunoglobulin M (IgM) which is a
pentamer
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The H chain and L chains are inter-connected and oriented in such ways that the
individual immunoglobulin unit (H2L2) appears in the shape of ‘Y’ or ‘T‘
...

The two heavy chains are covalently connected to each other through 1 to 5
disulphides (S-S) bonds
...

The binding of heavy chains with the light chain creates a heterodimer (HL)
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The H2L2 is thus the basic structure of an immunoglobulin
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They differ in their size, amino acid sequence, antigenicity, and the carbohydrate
content
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Light chain:
Each light chain consists of 220 to 240 amino acids with free ‘N’ and ‘C’ terminals
...

The light chains are structurally and chemically similar in all classes of
immunoglobulins
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The kappa and lambda chain differ in their amino acid sequence in their constant
regions (see below for constant and variable region of the immunoglobulin)
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In humans, 60% of the light chains are kappa and 40% is lambda
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In each chain, the amino terminal region is called Variable region (V region) and
the carboxy terminal is called Constant region (C region)
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The V and C region consists of repeating units of structural units called Domains
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Similarly, the variable and constant regions of light chains are designated as ‘VL’
and ‘CL’ respectively
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Each of these five
different heavy chains is
called an isotype
...

Each
heavy
chain
contains one variable
region (VH) and three or
four
constant
(CH)
regions
...

The heavy chains IgM and IgE have four constant regions designated as CH1,
CH2, CH3 and CH4
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The variable region is different in each class of immunoglobulins
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The variable regions of both heavy and light chains consist of three highly
variable regions called Hyper Variable Regions
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The ‘Fab’ region consists of a short stretch of only 5 – 10 amino acids
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Hinge Region of Immunoglobulin
The hinge region is a highly flexible amino acid stretch present in between the
CH1 and CH2 region of heavy chains of some immunoglobulin isotypes (IgG, IgA
and IgD)
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The heavy chains of IgM and IgE possess additional hinge-like domain with
approximately 110 amino acids
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The hinge region is rich in cysteine and proline amino acids
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