antibody. The detection antibody is conjugated to a solid coloured marker, usually
latex or colloidal gold, and is stored in a fibre pad which acts as a reservoir. The solid
phase has a layer of transparent plastic overlaying it leaving a very narrow gap which
will draw liquid by capillarity. The sample is applied to the reservoir pad through the
sample port where it can react with the conjugated antibody if the specific antigen is
present. The liquid then leaves the reservoir and travels up the solid phase pad to the
location of the trapping antibody. If the sample contains the specific antigen then it
will react to both the conjugated and trapping antibody. This results in a coloured line
if the sample is positive. If the sample is negative then no coloured line develops. The
system lends itself to multiplexing and up to three different antigens can be tested for
simultaneously with appropriate trapping antibodies and different coloured marker
particles. The technology has been applied to home pregnancy testing and various
other ‘self-diagnostic’ kits. Lateral flow devices are also used by police forces and
regulatory authorities for the rapid identification of recreational drugs.
7.6 Epitope mapping
Epitope mappingis carried out to establish where on the target protein the antibody
binds. The method works well with new monoclonal antibodies where it may be
necessary to know the precise epitope to which binding occurs. This however can
Reagent pad containing
antibody labelled
latex beads
Capture zone coated with
antibody which will bind
antigen/antibody/bead
complex
Sample moves down device
by capillarity until capture
zone is reached.
If antigen is present a
coloured line is formed
Port for sample application
Fig. 7.16Lateral flow device.
292 Immunochemical techniques
