Why are proteins transferred from gel to a membrane in western blot?

After gel electrophoresis, the separated proteins are usually transferred from the gel to a membrane for further detection. It is because conducting the protein detection on membrane is usually more sensitive and more cost effective.

• Proteins are not immobilized enough in gel, which may diffuse through the pores. The spreading of proteins in gel could end up with a mess after staining or at least compromise the detection sensitivity. However, on membranes, proteins can be well fixed since the mesh of membrane is not so diffusion friendly for large molecules like proteins. Therefore, the protein concentration is much higher on membrane compared to that on gel, resulting in a more sensitive detection as well as a more efficient reaction between antibody and antigen.
• Membrane can be stored for further probing with other different antibodies, whereas gels are difficult for long time storage.
• Although there are commercially available protocols for in-gel western blot, it usually requires a much larger amount of antibody to incubate the gel with, which is highly costly.