This exercise demonstrates the biological phenomenon of the formation of a precipitate when an antigen reacts with an antibody. The exercise can be used to illustrate the specificity of antigen-antibody reactions, showing that a precipitation reaction only occurs when an antibody reacts with the antigen that was used to induce the formation of the antibody. The exercise is also a general demonstration of diffusion.
If an antibody reacts with an antigen in a solution, the resulting complex precipitates. For example, if an extract from a plant with an unknown disease precipitates with antibodies to Tobacco mosaic virus (TMV), then it is concluded that the plant is infected with TMV. (Note, however, that because of the specificity of antigen-antibody reactions, this result does not exclude the possibility of the plant also being infected with additional viruses or other pathogens) Antigen-antibody reactions can be easily conducted and observed in gels made of agar or agarose. Antibodies and most antigens diffuse readily into such gels. (When introducing this exercise to your students, it may be a good time to review the topic of diffusion.) In single diffusion tests, antibody is incorporated into the gel and antigen is introduced into wells in the gel. Rings of precipitate form around the wells, and the diameters of these rings are proportional to the amount of antigen placed in the well. In double diffusion tests, antigen and antibody are placed into separate wells; they diffuse into the gel in all directions; where they interact, a precipitation reaction occurs. Each antigen-antibody reaction forms an independent precipitation reaction, called a precipitin boundary.