Carbohydrate Antigens
279
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Carbohydrate antigens are polysaccharides and glycocojugates of multiple structural
conFgurations that are able to evoke carbohydrate-speciFc antibodies and react with
these antibodies. Although recognition of carbohydrates as antigens began with
the study of microbial polysaccharides, the scope of investigation of carbohydrate
antigens has since been substantially extended. Carbohydrates are unsurpassed in
generating structural diversity and are prominent in surface display. They contain
valuable biological information and are characteristically recognizable by either
soluble or membrane-bound cellular proteins. In the higher eukaryotic species,
expression of cellular glycoconjugates, and especially their complex carbohydrate
structures, is frequently cell type–speciFc or tissue-speciFc. In microbes, many
sugar chains, including those displayed on the surface of a microbial cell and those
secreted outside it, have been recognized as ‘‘signatures’’ of speciFc pathogens.
Exploring the biological information content of sugar chains is the current focus
of postgenomic research. Carbohydrate antigen, as a classical immunological topic,
has gained renewed interest in the new millennium.
1
Classifcations
Carbohydrate
antigens
exist
in
multi-
ple structural conFgurations, including
polysaccharides,
glycoproteins,
glycoli-
pids, glycosphingolipids, glycosaminogly-
cans,
and
proteoglycans.
Given
their
origins, carbohydrate antigens were tra-
ditionally
classiFed
as
microbial
anti-
gens and alloantigens. Microbes produce
the former; the latter are expressed by
mammalian cells. Carbohydrates may be
‘‘complete-antigens,’’ such as microbial
polysaccharides, in that they are able
to induce reactive antibodies in certain
species. They may also be components
or antigenic determinants of glycocon-
jugates. Such carbohydrate moieties, or
oligosaccharides, are termed
haptens
in im-
munology. They determine the speciFcity
of an immune response but are not being
able to evoke an immune response in their
free, unconjugated forms. Consideration
of glycoconjugates as carbohydrate anti-
gens makes it necessary to further classify
carbohydrate antigens into two large im-
munological categories, the T-dependent
(TD) and T-independent (TI) forms of car-
bohydrate antigens.
Therefore, different methods have been
used to classify carbohydrate antigens.
Their
relationship
is
summarized
in
Table 1. We must emphasize that this table
is an attempt to illustrate how researchers
of different scientiFc disciplines ‘‘see’’ a
Tab. 1
Classifcation oF carbohydrate antigens.
TI-1
TI-2
TD
Microbial antigens
Polysaccharide
No
Yes
No
Lipopolysaccharide
Yes
No
No
Glycoprotein
No
No
Yes
Glycolipid
No
Yes
No
Allo- & xenoantigens
Glycoproteins
No
No
Yes
Glycolipids
No
Yes
No
Glycosaminoglycans
No
Yes
No
Proteoglycans
No
Yes
Yes (?)
Notes
: TI: T-independent; TD: T-dependent.
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