Polyclonal antibodies are antibodies that are derived from many different cells or cell lines, similar to the mixture of antibodies found in sera. Polyclonal antibodies are therefore a mixture of many different specificities. This is in contrast to monoclonal antibodies which are derived from one clone.
When researchers purchase a polyclonal antibody preparation, this is usually a mixture of antibodies of many different specificities to the same antigen.
Polyclonal antibodies are produced by immunizing a rabbit, mouse, goat, chicken, or other animal. The antigen (that you want to generate an antibody against) is injected into the animal along with a suitable adjuvant. This immunization of the animal results in the production of immunoglobulins specific to that antige, of which IgG (immunoglobulin G) is the most desirable. This is due to the binding properties of IgG, its high concentrations in serum, and because it is the easiest class of immunoglobulin to handle in the laboratory. The animal sera can be used directly, however it is usually purified to yield purer polyclonal antibody preparations.