<prompt> Which of the following is an example of precipitation reaction?Widal TestCoombs TestCounter Current immunoelectrophoresisWeil-Felix Test  <response>: Precipitation is a method for detecting an antigen-antibody reaction. When antigen and antibody combine in the proper propoions, a visible precipitate is formed. Optimum antigen-antibody ratios can be produced by allowing one to diffuse into the other, most commonly through an agar matrix immunodiffusion. Counterimmunoelectrophoresis CIE is a type of precipitation reaction where immunodiffusion carried out in an electrophoretic field. Both the speed and the sensitivity of immunodiffusion are improved by CIE. Ref: Ray C.G., Ryan K.J. 2010. Chapter 4. Principles of Laboratory Diagnosis of Infectious Diseases. In C.G. Ray, K.J. Ryan Eds, Sherris Medical Microbiology, 5e. 