ANTIGEN:
It is a substance which, when introduced into a body evokes immune response to produce
a specific antibody with which it reacts in an observable manner.
In most cases, antigen and immunogen terms are used interchangeably.
The antigens that are not immunogenic but can take part in immune reactions are termed
as haptens.
All molecules that are immunogenic are antigenic too, but all antigenic molecules cannot
be considered immunogenic.
Types of antigens
They may be classified as:
Complete Antigen
Incomplete Antigen (haptens):
Complex haptens
Simple haptens.
Complete antigens are substances which can induce antibody formation by themselves
and can react specifically with these antibodies.
Haptens are substances unable to induce antibody formation on its own but can become
immunogenic (capable of inducing antibodies) when covalently linked to proteins, called
carrier proteins.
However, haptens can react specifically with induced antibodies. These antibodies are
produced not only against the hapten but also against the carrier protein.
Types of haptens:
Complex haptens can combine with specific antibodies to form precipitates because they
are polyvalent e.g. capsular polysaccharide of pneumococci.
Simple haptens combine with specific antibodies but no precipitate is produced due to its
univalent character.
N/B: Precipitation occurs when antigen has two or more antibody combining sites.
Haptens are usually small molecules, but some high-molecular-weight nucleic acids are
haptens as well.
Other examples are many drugs like (penicillin) and catechol in the plant oil that causes
poison oak and poison ivy.
A hapten is a molecule that is not immunogenic by itself but can react with specific
antibody.
Haptens are not immunogenic because they cannot activate helper T cells.
The failure of haptens to activate is due to their inability to bind to MHC proteins; they
cannot bind because they are not polypeptides and only polypeptides can be presented by
MHC proteins.
Furthermore, haptens are univalent and therefore cannot activate B cells by themselves.
Although haptens cannot stimulate a primary or secondary response by themselves, they
can do so when covalently bound to a “carrier” protein.
In this process, the hapten interacts with an IgM receptor on the B cell and the hapten–
carrier protein complex is internalized.
A peptide of the carrier protein is presented in association with class II MHC protein to
the helper T cells.
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, The activated helper T cell then produces interleukins, which stimulate the B cells to
produce antibody specific to the hapten.
Superantigens
Are molecules that can interact with APCs and T-lymphocytes in a non-specific manner.
These antigens do not involve the endocytic processing as required in typical antigen
presentation.
Interaction of superantigens with MHC class II molecules of the APC and the Vβ domain
of the T-lymphocytes receptor activates a large number of T-lymphocytes than
conventional antigens leading to massive cytokine expression and immuno-modulation.
Examples: staphylococcal enterotoxins, toxic shock syndrome toxin, exfoliative toxins,
and some viral proteins.
2
It is a substance which, when introduced into a body evokes immune response to produce
a specific antibody with which it reacts in an observable manner.
In most cases, antigen and immunogen terms are used interchangeably.
The antigens that are not immunogenic but can take part in immune reactions are termed
as haptens.
All molecules that are immunogenic are antigenic too, but all antigenic molecules cannot
be considered immunogenic.
Types of antigens
They may be classified as:
Complete Antigen
Incomplete Antigen (haptens):
Complex haptens
Simple haptens.
Complete antigens are substances which can induce antibody formation by themselves
and can react specifically with these antibodies.
Haptens are substances unable to induce antibody formation on its own but can become
immunogenic (capable of inducing antibodies) when covalently linked to proteins, called
carrier proteins.
However, haptens can react specifically with induced antibodies. These antibodies are
produced not only against the hapten but also against the carrier protein.
Types of haptens:
Complex haptens can combine with specific antibodies to form precipitates because they
are polyvalent e.g. capsular polysaccharide of pneumococci.
Simple haptens combine with specific antibodies but no precipitate is produced due to its
univalent character.
N/B: Precipitation occurs when antigen has two or more antibody combining sites.
Haptens are usually small molecules, but some high-molecular-weight nucleic acids are
haptens as well.
Other examples are many drugs like (penicillin) and catechol in the plant oil that causes
poison oak and poison ivy.
A hapten is a molecule that is not immunogenic by itself but can react with specific
antibody.
Haptens are not immunogenic because they cannot activate helper T cells.
The failure of haptens to activate is due to their inability to bind to MHC proteins; they
cannot bind because they are not polypeptides and only polypeptides can be presented by
MHC proteins.
Furthermore, haptens are univalent and therefore cannot activate B cells by themselves.
Although haptens cannot stimulate a primary or secondary response by themselves, they
can do so when covalently bound to a “carrier” protein.
In this process, the hapten interacts with an IgM receptor on the B cell and the hapten–
carrier protein complex is internalized.
A peptide of the carrier protein is presented in association with class II MHC protein to
the helper T cells.
1
, The activated helper T cell then produces interleukins, which stimulate the B cells to
produce antibody specific to the hapten.
Superantigens
Are molecules that can interact with APCs and T-lymphocytes in a non-specific manner.
These antigens do not involve the endocytic processing as required in typical antigen
presentation.
Interaction of superantigens with MHC class II molecules of the APC and the Vβ domain
of the T-lymphocytes receptor activates a large number of T-lymphocytes than
conventional antigens leading to massive cytokine expression and immuno-modulation.
Examples: staphylococcal enterotoxins, toxic shock syndrome toxin, exfoliative toxins,
and some viral proteins.
2
