Monoclonal antibodies are produced by fusing an immune cell that is known to produce the antibody with a tumor cell that can grow in culture. Specifically, B-lymphocytes from the spleen are fused with myeloma cells by making the cell membranes more permeable. The fused hybrid cell (called hybridoma) will produce large amounts of antibodies that are exactly alike. Antibodies can be produced in cell culture or in animals. When the hybridoma cells are injected in mice (in the peritoneal cavity, the gut), they produce liquid-filled tumors. Inside these tumors is an antibody-rich fluid called ascites fluid. The small variation (if any) in recognizing the antigen helps to reduce side effects when the antibodies are used for medical treatment.
However, there are drawbacks to using monoclonal antibodies as opposed to polyclonals. Each lymphocyte cell produces antibodies that are specific not to an antigen, but to an epitope of that antigen. An epitope is a small piece of the antigen to which the antibody binds. Polyclonal antibodies bind to many epitopes of a given antigen, while monoclonals bind to a single epitope. In the processing of antibodies, certain binding capabilities are degraded. If the monoclonal antibody is susceptible to such degradation, it is useless. Polyclonals will still be useful even if certain epitope-binding species are degraded.