monoclonal antibodies may be better than polyclonal antibodies is certain clinical and research applications. But when used as the primary antibody in Western blotting or immunohistology procedures.
The idea that monoclonal antibodies are somehow inherently superior to polyclonal antibodies is utterly erroneous. Monoclonal does NOT mean monospecific. It just means that all of the antibody molecules in the supernatant are identical. There are LOTS of monoclonal antibodies out that are non-specific, just like there are for polyclonal antibodies. There is a very nice editorial commentary by Clifford Saper in Journal of Comparative Neurology (probably 10 years ago, maybe more) that discusses this issue very nicely.
Polyclonal antibodies often outperform monoclonal antibodies because the dominant antibody species in a polyclonal antiserum may have a much higher affinity for the antigen than monoclonal antibodies against the same antigen. Another factor is titer. The titer of polyclonal antibodies in a serum is often much higher than the antibody titer in a hybridoma supernatant.
Monoclonal antibodies bind a single epitope on your antigen, so only one molecule of antibody can bind to each antigen molecule, unless the epitope is repetitive.
On the other hand, polyclonal antibodies bind multiple epitopes on the same antigen, so several antibody molecules can in principle bind to each antigen molecule. That makes the signals usually higher when using polyclonal antibodies. If you have a good polyclonal antibody (highly specific for your antigen), sensitivity of antigen detection should be very high.
There is a second advantage of polyclonal antibodies in some conditions. A monoclonal antibody can work or not in western blot or immunohistochemistry depending on its epitope. For instance, if the antibody recognizes a conformational epitope, probably it will not detect denatured antigen in WB. On the other hand, if the epitope is affected by tissue processing procedures, then the antibody will not work in IHC. As polyclonal antibodies are mixtures of antibodies recognizing different epitopes, frequently (not always) they are useful for many techniques, because at least a fraction of the epitopes recognized is kept in a given experimental condition.
Additionally, If the antigen is polymorphic having different variants in nature, perhaps a monoclonal antibody is not able to recognize all of them because the epitope is not uniformly present. Polyclonal antibodies, recognizing multiple epitopes, tend to be less affected by such variations.
Despite the previous comments, in many cases a good monoclonal antibody is ideally suited for a given application. Its only necessary to find the right antibody for your purposes.
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