Why milk as a blocking reagent?

Answer 1

Most people use it because it's cheap. A blocking reagent is used to saturate any binding sites on your membrane/plate/whatever that will bind any old bit of protein just because it's protein, not because it's the thing you're looking for. Milk (or skimmed milk powder, which is what most people use) is just a big old load of protein. You'll find people will use BSA (bovine serum albumin) or other types of serum as blocks in other protocols. It all boils down to the same thing. Big old lot of protein that saturates all your non-specific binding sites before you start probing for your protein of interest. Means you should get a nice clean signal instead of a mucky one.

Answer 2

Milk is commonly used as a blocking reagent in immunoassays because it contains proteins like casein which can bind nonspecifically to free binding sites on the sample, preventing unwanted interactions with the antibody. This helps reduce background noise and increase the specificity of the assay by blocking sites that could potentially bind the primary antibody.