Re: [Histonet] Polyclonals vs Monoclonals

From:John Kiernan

A monoclonal antibody (MAB) binds to one tiny bit of the
antigen molecule, called an epitope, which typically
is a particular sequence of a few amino acids in
a particular conformation (shape). If the epitope 
is masked (for example, by other nearby protein 
molecules) a MAB cannot bind to it. If the same
epitope exists also in a different protein - not the
antigen to which the antibody was raised - then
the MAB will bind to the wrong antigen. (That is not
to say false-positives are abundant in immunohistochemistry
with MABs.)

A polyclonal antiserum contains numerous antibodies,
each with specificity for a different epitope of
the antigen, so there is a better chance of some
antibody molecules binding to the antigen that you
are seeking to stain. The positive immunostaining
results from summation of the different antibodies.

In an antiserum there will also be antibodies that
can bind to epitopes in proteins other than the
antigen of interest. These can give rise to
false-positive or background staining. There are
techniques for reducing unwanted immunostaining:
notably absorption of the antiserum with a tissue
powder that does not contain the antigen, and a
simple chromatographic procedure called affinity
purification. The leaflet with a commercially
supplied antiserum should tell you if these
procedures have been applied to the product.

In short, one cannot in general prefer monoclonals
or polyclonals. Either may be better for a
certain job. It's important to do the classical
control procedures when doing imunohistochemistry,
aspecially with a combination of antibody and
tissue that you haven't worked with before.

                  John Kiernan
                  London, Canada.
Rena wrote:
> I know that monoclonals are more specific than polyclonals. The question
> is which is preferred. Do you have both and use the polyclonal as a
> screening antibody, then use the monoclonal?  Do you prefer monoclonals
> or polyclonals and why?
> Thank you
> Rena Fail
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