In addition to their use in passive immunity, antibodies are also commercially prepared for use in research and clinical laboratory tests. In the past, antibodies were obtained by chemically purifying a specific antigen and then injecting this antigen into animals. Since an antigen typically has many different antigenic determinant sites, however, the antibodies obtained by this method were polyclonal; they had different specificities. This decreased their sensitivity to a particular antigenic site and resulted in some degree of cross-reaction with closely related antigen molecules.
Monoclonal antibodies, by contrast, exhibit specificity for only one antigenic determinant site. In the preparation of monoclonal antibodies, an animal (frequently, a mouse) is injected with an antigen and subsequently killed. B lymphocytes are then obtained from the animal's spleen and placed in thousands of different in vitro incubation vessels. These cells soon die, however, unless they are hybridized with cancerous multi ple myeloma cells. The fusion of a B lymphocyte with a cancerous cell produces a potentially immortal hybrid that undergoes cell division and produces a clone, called a hybridoma. Each hybridoma secretes large amounts of identical monoclonal antibodies. From among the thousands of hybridomas produced in this way, the one that produces the desired antibody is cultured for large-scale production and the rest are discarded (fig. 15.24).
The availability of large quantities of pure monoclonal antibodies has resulted in the development of much more sensitive clinical laboratory tests (for pregnancy, for example). These pure antibodies have also been used to pick one molecule (the specific antigen interferon, for example) out of a solution of many molecules so as to isolate and concentrate it. In the future, monoclonal antibodies against specific tumor antigens may aid the diagnosis of cancer. Even more exciting, drugs that can kill normal as well as cancerous cells might be aimed directly at a tumor by combining these drugs with monoclonal antibodies against specific tumor antigens.
468 Chapter Fifteen
Test Yourself Before You Continue
1. Describe three methods used to induce active immunity.
2. Using graphs to illustrate your discussion, explain the characteristics of the primary and secondary immune responses.
3. Explain the clonal selection theory and indicate how this theory accounts for the secondary response.
4. Define immunological tolerance, and explain mechanisms that may be responsible for tolerance to self-antigens by T and B lymphocytes.
5. Describe passive immunity and give examples of how it may occur naturally and how it may be conferred by artificial means.
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