288
Carbohydrate Antigens
however, signifcant diFFerences. Prokar-
yotes lack internal membrane-bound or-
ganelles, such as a nucleus, endoplasmic
reticulum (ER), and Golgi apparatus. They
process oligosaccharides on the outside
oF the cell membrane in the so-called
extracellular space. The sugar chains dec-
orating the glycoproteins oF microbes oF
various strains or species are quite di-
verse in their structures. Many oF them
are selectively or specifcally expressed
by certain strains and/or species. There
are, however, instances where bacteria-
produced oligosaccharides share similarity
with eukaryotic sugar chain structures.
As discussed above, a documented ex-
ample is the structural similarity between
the
α
(2–8)-polysialic acid capsular polysac-
charide oF human pathogens and the
sialic acid structures oF human brain tis-
sues. By mimicking host carbohydrates,
the pathogen evades detection by the
immune system, which recognizes it as
part
oF
‘‘selF.’’
This
has
been
recog-
nized as a strategy by which a num-
ber oF pathogens evade host immune
recognition.
Microheterogeneity in carbohydrates is
another documented phenomenon that
can
be
attributed
to
the
characteris-
tics oF sugar chain biosynthesis. Sharply
contrasted with the template-dependent
mechanism oF nucleic acid and protein
synthesis, there is no rigorous quality-
control mechanism in the biosynthesis
oF sugar chains. A polysaccharide may
thus not only have a main sugar chain
composed oF a type oF repetitive struc-
ture but also contain a Few diFFerent
minor structural moieties as well. Im-
munologically, the Former may Form a
dominant
antigenic
determinant
oF
a
polysaccharide. The latter may generate
a minor antigenic determinant oF the
antigen.
3
Sugar Chains as Antigenic Determinants
As
discussed
above,
microbial
carbo-
hydrate antigens oF multiple structural
confgurations
have
been
recognized.
Polysaccharides and lipopolisaccharides
are widely produced by many bacterial
species. Glycoproteins are expressed and
displayed on the surFace oF virions in
almost all known viral species inFecting
mammals. Viruses take advantage oF the
cellular machineries oF carbohydrate syn-
thesis to produce sugar chains and glyco-
proteins. Parasites oF mammals are higher
eukaryote species and have their own en-
zymatic systems For the biosynthesis oF
carbohydrate molecules and protein gly-
cosylation, generating diverse sugar struc-
tures. Presence oF glycoproteins in some
bacterial species has been also recently rec-
ognized. The oligosaccharide chains that
defne the antigenic specifcities oF these
microbial antigens are their antigenic de-
terminants.
3.1
Sizes of Antigenic Determinants
An antigenic determinant is a part oF an
antigen that is bound by and is comple-
mentary to the combining site oF a specifc
antibody. It is the key element For host
recognition and immune response. The
antigenic determinants oF a carbohydrate
antigen are generally its oligosaccharide
chains in certain size, shape, and con-
Formation. Given this widely accepted
defnition, it is possible to estimate the
size oF a carbohydrate-based antigenic de-
terminant by characterizing the specifc
interaction between carbohydrate ligand
and its antibody-combining sites. The exact
sizes and shapes oF an antigenic deter-
minant can be, however, visualized by
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