An
antigen has been defined as any substance which, when introduced parenterally
into the body of an organism, stimulates the production of an antibody with
which it reacts specifically and in an observable manner.
The
word ‘parenteral’ is used in the
definition because orally administered antigens are usually denatured by
digestive enzymes and their antigenicity destroyed, so that no antibody
formation takes place. When given parenterally, antigens do not undergo any
such inactivation and can induce antibody production.
The
word ‘specifically’ in the
definition is important as specificity is the hallmark of all the immunological
reactions. An antigen introduced into the body reacts only with those
particular immunocytes (B or T lymphocytes) which carry specific marker for
that antigen and which produce and antibody or cells complementary to that
antigen only.The antibody so produced will react only with that particular
antigen and with no other.
The
two attributes of antigenicity are – (a) introduction of an immune response,
and (b) specific reaction with antibodies or sensitized cells. Based on the
ability to carry out these two functions, antigens may be classified into
different types –
1. Complete antigen – A complete
antigen is able to induce antibody formation and produce specific and
observable reaction with the antibody so produced.
2. Haptens – Haptens are substances
that are incapable of inducing antibody formation by themselves but can react
specifically with antibodies. Haptens become immunogenic on combining with a
larger molecule carrier. Haptens may be simple or complex. Complex haptens can precipitate with the specific antibodies, while
simple haptens are non-precipitating.
The
smallest unit of antigenicity is known as the antigenic determinant or epitope.
The epitope is that small area on the antigen, usually consisting of four or
five amino acids or monosaccharide residues possessing a specific chemical
structure, electrical charge and steric configuration, capable of sensitizing
an immunocyte and of reacting with its complementary site on the specific
antibody or T cell receptor.
BIOLOGICAL
CLASSES OF ANTIGENS
Depending
on their ability to induce antibody formation, antigens are classified as T cell dependent (TD) and T cell independent (TI) antigens.
Antibody production is the property of B lymphocytes. For the full expression
of this function, however, the cooperation of T lymphocytes is necessary. Some
antigens can directly stimulate antibody production by B cells, without the
apparent participation of T cells. Such antigens are called TI antigens. Others
that require T cell participation to generate an immune response are called TD
antigens.
Several
important differences exist between TI and TD antigens. These are –
|
TI
Antigens
|
TD
Antigens
|
|
1.
TI antigens are structurally simple, being composed of a limited number of
repeating epitopes, as in the case of the pneumococcal capsular
polysaccharide, bacterial lipopolysacc-haride and the flagellar protein
flagellin.
|
1.
TD antigens on the other hand are structurally more complex, such as
erythrocytes, serum prot-eins, and a variety of protein-hapten complexes.
|
|
2.
Their immune response is critically dose dependent. Too little is
non-immunogenic, while too much results in immunological tolerance rather
than immunity.
|
2.
They are immunogenic over a wide dose range and do not cause tolerance
readily.
|
|
3.
Their antibody response is usually limited to IgM and IgG3.
|
3.
They include the full gamut of immunoglobulin isotypes – IgM, IgG, IgA and
IgE.
|
|
4.
They do not show immunological memory.
|
4.
They show immunological memory.
|
|
5.
TI antigens do not appear to require preliminary processing by macrophages.
They are metabolized very slowly and remain in the body for long periods.
|
5.
They are rapidly metabolized in the body.
|
*************
No comments:
Post a Comment