Why you use skim milk to block the membrane in western blotting?

Answer 1

The membrane binds proteins unspecifically (it will bind any).

Western blotting uses antibodies to probe the proteins bound to the membrane for the one you are testing for. The antibodies bind specifically to a particular protein that you have chosen and contain a label to allow identification of them on the membrane. In this way you can see whether or not the membrane, and therefore the tissue you are testing, contains a particular protein.

Antibodies are proteins. This means the antibody would bind anywhere on the membrane, as well as on the protein you're looking for, rendering the results worthless (it will ALWAYS show positive). To stop the antibodies binding to the membrane, so that they only bind to the protein (if it exists) the membrane must be blocked (completely filled with protein) so that there is nowhere on the membrane for the antibody to bind.

Since milk contains a generic mix of a relatively high concentration of known proteins, it is used to block the parts of the membrane that don't already have protein on so that the antibodies won't bind there. As it is very cheap and readily available, it is a good source of protein to use. It is unlikely that a Western blot would be needed for any protein that is contained in milk, but if that is the case an alternative mixture can be used.