Most antigens offer multiple epitopes (antigenic determinants) and therefore induce proliferation and differentiation of a variety of B-cell clones, each derived from a B-cell that recognizes a particular epitope. Polyclonal antibodies represent a mixture of antibodies synthesized by variety of such B-lymphocyte (B-cell) clones.
The resulting polyclonal antibodies in the antiserum are heterogenous, comprising a mixture of antibodies, each specific for one epitope (Fig. 41.9). Polyclonal antibodies show clear advantages for the organism in vivo as they facilitate the localization, phagocytosis, and complement-mediated lysis of antigen.
Since the polyclonal antibodies react with multiple epitopes on the surface of the antigen, they will be more tolerant of minor changes in the antigen (e.g., polymorphism, heterogeneity of glycosylation, or slight denaturation) than will be the monoclonal antibodies that are homogenous.
Depending upon the antigen that is used to create the antibody, one may use polyclonal antibodies to identify proteins of high homology to the immunogen protein or to screen for the target protein in tissue samples from species other than that of the immunogen.
Along the same lines, it is especially important when working with polyclonal antibodies to educate one’s self about the immunogen that has been used for the production of the polyclonal antibody and the potential for desired cross reactivity within one’s sample. Peptide immunogens are often used to generate polyclonal antibodies that target unique epitopes, especially for protein families of high homology.
Some useful properties of polyclonal antibodies are that:
(1) They often recognize multiple epitopes, making them more tolerant of small changes in the nature of the antigen,
(2) They are often the preferred choice for detection of denatured proteins,
(3) They may be generated in a variety of species (e.g., sheep, goat, rabbit, donkey, chicken, etc.) providing the users many options in experimental design,
(4) They are sometimes used when the nature of the antigen in an untested species is not known and
(5) They target multiple epitopes and so they generally provide more robust detection.
Polyclonal antibodies are often available in serum or antiserum that refers to the blood from an immunized host from which the clotting proteins and RBCs have been removed. The, antiserum, as its name suggests, still possesses antibodies/immunoglobulin’s of all classes as well as other serum proteins.
In addition to antibodies that recognize the target antigen, the antiserum also contains antibodies to various non-target antigens that can sometimes react non-specifically in immunological arrays.
For this reason, raw antiserum is often purified to eliminate serum proteins and to enrich the fraction of antibody that reacts with the target antigen. Table 41.7.compares the properties of polyclonal antibodies with the properties of monoclonal antibodies (Mabs).