Most antigens offer multiple epitopes and therefore
induce proliferation and differentiation of a variety of
B-cell clones, each derived from a B cell that recognizes
a particular epitope. The resulting serum antibodies are
heterogeneous, comprising a mixture of antibodies, each
specific for one epitope. Such a polyclonal antibody
response facilitates the localization, phagocytosis, and
complement-mediated lysis of antigen; it thus has clear
advantages for the organism in vivo.
Unfortunately, the antibody heterogeneity that increases
immune protection in vivo often reduces the efficacy of an
antiserum for various in vitro uses. For most research,
diagnostic, and therapeutic purposes, monoclonal
antibodies, derived from a single clone and thus specific
for a single epitope, are preferable.