The antibodies produced against most foreign antigens are composed of a family of immunoglobulins, a family composed of members that are of a number that often reflects the size/complexity of the molecule that stimulates their production. In other words, such responses involve the activation of a “polyclonal” B lymphocyte population. The antibody products of the B cells, although all capable of binding the original antigen, bind at various immunogenic sites (epitopes) on that antigen. Such differences in antigen-binding fine specificity is determined by amino acid residues in the antibody variable region domains found associated with the antigen combining site and tend to have a complimentary biochemistry with the molecule for which they are intended to interact. Furthermore, in addition to amino acid differences that dictate the isotypes and allotypes of antibody molecules, differences in the amino acids that compose the variable regions can produce differences in net charge of particular antibody molecules; thus, families of polyclonal antibodies, all reactive with the same antigen but with different fine specificities, can be separated and, as shown below, purified based on their isoelectric points by preparative isoelectric focusing (pIEF).
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