Antibodies are immunological proteins that play fundamental roles in host defense against infectious agents such as viruses, bacteria and fungi. The immunological functions of these Y-shaped proteins are determined by their ability to bind antigens, the molecules released by or found upon cells that stimulate an immune response.
Beyond their roles in host immunity, antibodies prove to be valuable for research and therapeutic purposes too. Two types of antibodies – polyclonal and monoclonal – provide research scientists with distinct ways to detect or quantify target antigens largely due to differences in specificity and affinity. In this article, we describe polyclonal and monoclonal antibodies in more detail and consider how each type may be used for various applications.
Population of antibodies
Recognize a single epitope on a target antigen
Recognize multiple epitopes on a target antigen
Better for protein quantitation
High for low-quantity proteins
Lower due to high specificity
Higher due to biophysical diversity
Cost to produce
Single lineage of stimulated B cells; hyridomas
Multiple lineages of stimulated B cells
Two of the key features that define the usefulness of polyclonal and monoclonal antibodies involves their respective specificity and sensitivity to an antigen. The specificity of an antibody is determined by the relative affinity between its binding domain and the target antigen while other molecules are present. The exploitation of this specificity is essential for immunological researchers and clinicians since many applications utilize polyclonal and/or monoclonal antibodies to specifically detect target molecules. In conjunction with antigen specificity, the sensitivity of an antibody is an important parameter that determines its usefulness in the lab.
High sensitivity antibodies are well suited for diagnostic applications5 like immunoprecipitation, western blot and enzyme-linked immunosorbent assay (ELISA) due to their ability to recognize low levels of target antigen. Immunoprecipitation is an analytical technique that isolates an antigen from a mixture by using an antibody that specifically binds the antigen. The resulting antigen-antibody complexes can then be further processed by an immobilized antibody or magnetic beads, which allows the antigen-antibody complexes to be separated from the mixture prior to analysis.
Like immunoprecipitation, ELISAs6 can use polyclonal or monoclonal antibodies to detect a target antigen in solution. Indirect and sandwich ELISAs are two immunoassay forms that use both types of antibodies throughout their procedures: monoclonal antibodies are typically applied first (primary) due to their high specificity to the target antigen; polyclonal antibodies are more useful as the secondary reagent due to their ability to amplify a low signal with high sensitivity7. Beyond ELISAs, antibodies are useful for detection techniques like immunohistochemistry and flow cytometry since they can detect antibody-antigen constructs within a complex tissue or cell with high resolution.
Confirmation of antibody specificity and sensitivity by manufacturers allow immunoassays like ELISAs to take on different forms depending on how the antigens and antibodies are used, which make these tests highly versatile. For example, an immunoassay that requires a highly sensitive test to detect an unknown pathogen would benefit from polyclonal antibodies due to their ability to recognize multiple epitopes on an antigen. If a pathogen or antigen has previously been characterized, then it may be more appropriate to use monoclonal antibodies in downstream applications for further characterization.
Outside of the laboratory, antibodies can be used to enhance, mimic or restore the immune system’s ability to attack disease targets. With some exceptions, monoclonal antibodies are better suited for therapeutic purposes than polyclonal antibodies due to their homogeneity, high specificity to a single epitope and their low degree of cross-reactivity8. These properties are especially important for effective cancer therapies since tumor cells are capable of evading or blocking the immune system. Some of the ways monoclonal antibody therapies function against cancer cells include mechanisms that involve direct binding to induce cell death and indirect mechanisms that block tumors from growth factors and blood supply.9