in culture; the cells are then screened for the antibody in question. These hybridomas are separately grown, leading to pure antibodies being made. As you can imagine, this is expensive and time-consuming. What is the purpose of making monoclonal antibodies? Part of the purpose is to turn mouse antibodies into human monoclonal antibodies. Mouse antibodies will be recognized as foreign by human so they will be neutralized by the human immune system. Genes can be manipulated that make a hybrid antibody that will be mostly human in origin. These are used to treat certain cancers by making antibodies against the cancer cell. Because of the expense, it is necessary to find an alternative way of making hybrid antibodies. Genetically-engineered plants can be used to make plant-related antibodies that can be humanized. These might be cheap enough to be made for use in treating common infectious diseases. Animal-derived monoclonal antibodies are just too expensive to make otherwise. Making these types of antibodies in plants is called making plantibodies.
DETECTION OF ANTIGEN-ANTIBODY COMPLEXES In vitro assays involve the detection of antibody-antigen complexes in a test tube or other medium outside of the body. When an antigen and antibody can be seen, it is referred to as a precipitin. In this type of reaction, an antibody and antigen are mixed together. If they combine, they precipitate out of solution, leading to a precipitin. Most of these tests are done using polyclonal antibodies, which form a precipitate more easily. The ratio of antibodies and antigens is important in a precipitation reaction. If there are too many antibodies, they will not combine well to make a precipitate. If there are too many antigens, the amount of precipitation will also decrease. There is an equivalence zone that provides an optimal ratio between antigens and antibodies in the reaction. A related test is called a precipitin ring test. It can tell the relative amounts of antigen and antibody in a sample. Test tubes are made that have the same amount of antigen in them. Glycerol is added to the antigen; then there is serial dilution of the antiserum. The glycerol allows antigen-antibody complexes to form just at the interface between the 241
