immunodiffusion /-difyo̅o̅″zhən/ [L, immunis + diffundere, to spread] , a technique for the identification and quantification of any of the immunoglobulins. It is based on the presence of a visible precipitate that results from an antigen-antibody combination under certain circumstances. Gel diffusion is a technique that involves evaluation of the precipitin reaction in a clear gel, seen when an antigen placed in a hole in the agarose diffuses evenly into the medium. An obvious ring forms where the antigen meets the antibody. Electroimmunodiffusion is a gel diffusion to which an electrical field is applied, accelerating the reaction. Double gel diffusion is a technique that permits identification of antibodies in mixed specimens. In an agar plate antigen is placed in one well, antibody in another. Antigen and antibody diffuse out of their wells. In mixed antigen specimens each antigen-antibody combination forms a separate line; observation of the location, shape, and thickness of a line permits identification and quantification of the antibody.
