Download Chapter One Introduction And Literature Review

L.2
2016-2017
Microbiology/Immunology
Asst.Prof.Dr. Ifad Kerim Al-Shibly
ANTIGENS & IMMUNOGENS
Immunogen:
A substance that induces a specific immune response.
Antigen (Ag):
A substance that reacts with the products of a specific immune response. It may
be specifically bound by an antibody (Ab) molecule or T cell receptors (TCR).
There are some Ag do not induce immune response. So all immunogens are
antigens, but not all antigens are immunogens .
Hapten:
A substance that is non-immunogenic but which can react with the products of a
specific immune response. Haptens are small molecules (MW less than 1000 D such
as nickel, chromate, and some drugs) which could never induce an immune response
when administered by themselves
but which can when coupled to a
carrier molecule. Free haptens,
however, can react with products
of the immune response after
such products have been elicited.
Haptens have the property of
antigenicity
but
not
immunogenicity.
FACTORS INFLUENCING IMMUNOGENICITY
A. Factors related to the immunogen:
1. Foreignness: The immune system normally discriminates between self and
non-self such that only foreign molecules are immunogenic.
2. Size: There is no absolute size above which a substance will be immunogenic.
However, in general, the larger the molecule the more immunogenic it is likely to be.
The most potent immunogens are proteins with a MW 100000 D. Substances smaller
than 10000 D are not usually immunogenic.
3. Chemical Composition: Usually proteins are very potent immunogens.
Polysaccharides can also be immunogenic to a lesser extent when compared with
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L.2
2016-2017
Microbiology/Immunology
Asst.Prof.Dr. Ifad Kerim Al-Shibly
proteins. Pure lipids are not immunogenic. In general, the more chemically complex
the substance is the more immunogenic it will be.
4. Physical form: In general particulate antigens are more immunogenic than
soluble ones and denatured antigens more immunogenic than the native form.
5. Degradability: Antigens that are easily phagocytosed are generally more
immunogenic. This is because for most antigens (T-dependant antigens, see below)
the development of an immune response requires that the antigen be phagocytosed,
processed and presented to helper T cells by an antigen presenting cell (APC).
B. Factors related to the biological System:
1. Genetic Factors: The ability to respond to a certain Ag is genetically
predetermined. Some substances are immunogenic in one species but not in another.
Pure polysaccharides are immunogenic when injected into human, but when injected
into the guinea pigs are not immunogenic. In human, immune response to a particular
Ag may differ from individual to another one. Similarly, some substances are
immunogenic in one individual but not in others. The species or individuals may lack
or have altered genes that code for the specific immunological response.
2. Age: Age can also influence immunogenicity. Usually the very young and the
very old have a diminished ability to mount an immune response to an immunogen.
C. Factors related to the method of administration:
1. Dose: The dose of administration of an immunogen can influence its
immunogenicity. There is a dose of antigen above or below which the immune
response will not be optimal. If tiny or too large amount of immunogen is used, the
host may fail to response.
2. Route: Generally the subcutaneous route is better than the intravenous or
intragastric routes. The route of antigen administration can also alter the nature of the
response.
3. Adjuvants: Substances that can enhance the immune response to an
immunogen are called adjuvants. The use of adjuvants, however, is often hampered by
undesirable side effects such as fever and inflammation. The mechanisms by which
adjuvant can enhance the immune response include:1-prolonging retention i.e,
increase the time of exposure of host to immunogen, 2-increasing the effective size, 3promoting immunological activities of immune cells, and 4- stimulating the influx of
immune cells to the site of administration.
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L.2
2016-2017
Microbiology/Immunology
Asst.Prof.Dr. Ifad Kerim Al-Shibly
TYPES OF ANTIGENS
A.
T-independent Antigens: T-independent antigens are antigens which can
directly stimulate the B cells to produce antibody without the requirement for T cell
help. In general, polysaccharides are T-independent antigens. The responses to these
antigens differ from the responses to other antigens.
B. T-dependent Antigens: T-dependent antigens are those that do not
directly stimulate the production of antibody without the help of T cells. Proteins are
T-dependent antigens.
ANTIGENIC DETERMINANTS:
An epitope, also known as
antigenic determinant, is the
part of an antigen that is
recognized by the immune
system,
specifically
by
antibodies, B cells, or T cells.
For example, the epitope is the
specific piece of the antigen
that an antibody binds to. The
part of an antibody that binds
to the epitope is called a
paratope. Although epitopes
are usually non-self proteins,
sequences derived from the host that can be recognized (as in the case of autoimmune
diseases) are also epitopes.
The antigenic determinants are created by the primary sequence of residues in the
polymer and/or by the secondary, tertiary or quaternary structure of the molecule.
A. Determinants recognized by B cells: Antigenic determinants recognized by
B cells and the antibodies secreted by B cells are small and limited to approximately
4-8 residues (amino acids and or sugars). Usually the antigenic determinants are
limited to those portions of the antigen that are accessible to antibodies.
B. Determinants recognized by T cells: Antigenic determinants recognized by
T cells are created by the primary sequence of amino acids in proteins. T cells do not
recognize polysaccharide or nucleic acid antigens. This is why polysaccharides are
generally T-independent antigens and proteins are generally T-dependent antigens.
The determinants need not be located on the exposed surface of the antigen since
recognition of the determinant by T cells requires that the antigen be proteolytically
degraded into smaller peptides. Free peptides are not recognized by T cells, rather the
peptides associate with molecules coded for by the major histocompatibility complex
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L.2
2016-2017
Microbiology/Immunology
Asst.Prof.Dr. Ifad Kerim Al-Shibly
(MHC) and it is the complex of MHC molecules + peptide that is recognized by T
cells.
SUPERANTIGENS:
When the immune system encounters a conventional T-dependent antigen, only a
small fraction (1 in 104 -105) of the T cell population is able to recognize the antigen
and become activated (monoclonal response). However, there are some antigens
which polyclonally activate a large fraction of the T cells (up to 25%). These antigens
are called superantigens Examples of superantigens include: Staphylococcal
enterotoxins (food poisoning), Staphylococcal toxic shock toxin (toxic shock
syndrome), Staphylococcal exfoliating toxins (scalded skin syndrome) and
Streptococcal pyrogenic exotoxins (shock). Although the bacterial superantigens are
the best studied; there are
superantigens
associated
with viruses and other
microorganisms as well. The
diseases associated with
exposure to superantigens
are, in part, due to hyper
activation of the immune
system
and
subsequent
release of biologically active
cytokines by activated T
cells.
DETERMINANTS RECOGNIZED BY THE INNATE IMMUNE SYSTEM:
Determinants recognized by components of the innate (nonspecific) immune
system differ from those recognized by the adaptive (specific) immune system.
Antibodies, and the B and T cell receptors recognize discrete determinants and
demonstrate a high degree of specificity, enabling the adaptive immune system to
recognize and react to a particular pathogen. In contrast, components of the innate
immune system recognize broad molecular patterns found in pathogens but not in the
host. Thus, they lack a high degree of specificity seen in the adaptive immune system.
The broad molecular patterns recognized by the innate immune system have been
called PAMPS (pathogen associated molecular patterns) and the receptors for PAMPS
are called PRRs (pattern recognition receptors). A particular PRR can recognize a
molecular pattern that may be present on a number of different pathogens enabling the
receptor to recognize a variety of different pathogens.
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