280
Carbohydrate Antigens
common object and work together to bet-
ter understand the object – carbohydrates.
When a specifc molecule is involved, ex-
periments must be conducted to learn its
immunological properties. Diversity oF car-
bohydrate antigens and the complexity oF
host immune systems have made it diF-
fcult to summarize a general scheme
For the antibody responses induced by
carbohydrate antigens. Seeing exceptions
regarding the TD and TI characteristics
oF an antigen From a given class may not
be surprising.
1.1
Microbial Antigens
Microbial carbohydrate antigens include
polysaccharides, glycolipids, and glyco-
proteins.
Polysaccharides
may
be
cell
wall components oF microorganisms, such
as
lipopolysaccharides (LPS)
in
gram-
negative bacteria, or capsules in gram-
positive
bacteria,
known
as
capsular
polysaccharides
. They may also be secreted
as exopolysaccharides, Forming slime-like
amorphous materials to trap microbial
cells in a viscous matrix. These polysac-
charides are all located on the cell surFace
and are, thereFore, oF importance in the
recognition and immune response oF a
higher organism to microbial inFections.
Glycoproteins were recently recognized in
a number oF bacterial species. By contrast,
they were Found essentially in almost all
membrane-bound or secretory proteins oF
viruses and many parasites.
1.2
Alloantigens and Xenoantigens
Eukaryotic cells also express carbohydrate
antigens,
notably
alloantigens
oF
the
human
blood
group
systems
and
xenoantigens expressing
α
-Gal epitopes.
T
h
eF
o
rm
e
ri
sa
na
l
l
o
g
e
n
i
cm
a
r
k
e
ro
F
human red blood cells and is responsible
For the rejection oF an allogenic blood
transFusion. The latter is a cross-species
barrier in organ transplantation and is
known as the main cause oF human
immune
rejection
oF
a
donor
pig’s
kidney. These antigens are present as
glycoconjugates
oF
diFFerent
molecular
c
o
n
f
g
u
r
a
t
i
o
n
ss
u
c
ha
sg
l
y
c
o
p
r
o
t
e
i
n
s
and glycolipids. The carbohydrate chains
attached
to
either
proteins
or
lipids
serve as antigenic determinants. Some
carbohydrate
structures
are
composed
oF
more
than
two
diFFerent
sugar
residues
and
have
Further
chemical
modifcations at specifc positions. Such
structures are called
complex carbohydrates
.
They
are
systematically
assembled
by
sets
oF
enzyme
systems
via
complex
biosynthesis processes. The expression oF
b
l
o
o
dg
r
o
u
ps
u
b
s
t
a
n
c
e
sA
,B
,H
,a
n
d
Lewis (Le) appears to be developmentally
and diFFerentiationally regulated. Atypical
expression oF such substances may lead
to
the
Formation
oF
tumor-associated
antigens.
1.3
T-dependent and T-independent Antigens
Protein antigens Fail to elicit antibody
responses in
athymic
mice and
nude
(
/
) mice. Polysaccharides and other
macromolecules with repetitive antigenic
determinants can induce unimpaired an-
tibody responses in these mice. These
observations establish two broad cate-
gories oF antigens, T-dependent (TD) and
T-independent (TI). TI antigens are Fur-
ther divided into type I (TI-I) and type
II (TI-II), based on the ability to elicit
antibodies in the CBA/N mouse strain,
with an X-chromosome linked immunod-
efciency (
xid
). Lipopolysaccharides (LPS)
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