There
are
two
main
procedures
in
use,
the
direct
and
the
indirect
methods.
The
direct
method
consists
of
bringing
fluorescein
tagged
antibodies
into
contact
with
a
smear
fixed
on
a
slide.
If
antigens
are
present
in
the
smear,
they
will
bind
the
tagged
antibody.
Excess
anti
body
is
washed
from
the
slide,
and
the
slide
is
examined
with
a
fluorescent
microscope.
The
site
of
union
of
the
labelled
antibody
with
its
antigen
can
be
seen
by
fluorescence
against
a
dark
background.
The
indirect
method
can
be
used
both
for
detecting
specific
antibodies
in
sera
or
other
body
fluids,
and
also
for
identifying
antigens.
In
the
serodiagnosis
of
syphilis,
for
example,
Treponema
pallidum
fixed
to
a
slide
is
flooded
with
the
unlabelled
antiserum
to
be
tested
for
anti
body.
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