Carbohydrate Antigens
281
of gram-negative bacteria, which induce
antibodies in such strains, are the TI-I anti-
gens; capsular polysaccharides of gram-
positive bacteria and exopolysaccharides
that induce no response in
xid
strains are
classiFed as TI-II antigens.
Glycoproteins are TD antigens; other
forms of glycoconjugates, including glycol-
ipids, Glycosphingolipids, glycosamino-
glycans, and proteoglycans, are generally
T-independent in nature. The carbohy-
drate chains of glycosaminoglycans and
proteoglycans are composed of repeats of
disaccharide units and are relatively large.
Their antigenic reactivities may vary sig-
niFcantly among molecules and depend
on the structure of a speciFc preparation
and the animal species used for immuniza-
tion. Glycolipids are a unique category of
antigens since their hydrophobic lipid tails
may integrate into cellular membranes of
distinct cell types while the hydrophilic
sugar chains are displayed on the surfaces.
There are many documented examples
to prove that molecules with such struc-
tural conFgurations are potent antigens
and vaccines.
2
Diversity of Carbohydrates
2.1
Composition, Sequences, and Linkages
Carbohydrates are unique in their struc-
tural diversity. The structure of a sugar
chain is determined by the composition
and sequence of its sugar residues, as well
as by their glycosidic linkages. This is dif-
ferent from nucleic acids and proteins that
are synthesized linearly in cells with a sin-
gle type of covalent bond to bridge their
monomers. Carbohydrates can be linked
together in multiple ways and can thereby
generate different structures with identical
residue compositions and sequences.
Monosaccharides are the basic build-
ing blocks of carbohydrate structures.
They can be linked by means of either
α
-o
r
β
-anomeric linkages at each join-
ing point, whereby the free hydroxyl on
given monosaccharide links an adjacent
monosaccharide (±ig. 1). Unlike proteins
that are connected solely by a peptide
bond, carbohydrates utilize many possible
Fig. 1
α
-or
β
-anomeric linkages of
carbohydrates. The diagram illustrates
the two kinds of linkages that may exist
between at least two monosaccharides
or sugar residues. These linkages may
be either
α
-or
β
-anomeric where the
free hydroxyl on one monosaccharide
connects to an adjacent
monosaccharide.
O
H
HO
H
CH
2
OH
H
OH
H
OH
H
5
1
2
4
6
O
H
H
CH
2
OH
H
OH
OH
H
OH
H
3
O
a
-Glycosidic linkage
a
-Maltose
O
H
HO
H
CH
2
OH
H
OH
H
OH
H
O
H
H
CH
2
OH
OH
H
OH
H
OH
H
b
-Glycosidic linkage
O
b
-Cellobiose
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