I have a question on immunostaining, about the step prior to primary Ab staining. By incubating the cells with a blocking agent such as normal serum derived from the same species as the secondary Ab, we can reduce non-specific staining (because the normal serum will coat the cell surface?), as I was told. My questions are:

1. If the normal serum can prevent the binding of the primary Ab to non-cognate proteins, will it not prevent its binding to cognate Ag as well?

2. Is the binding of primary Ab to non-cognate proteins on the cell surface not just as likely as its binding to non-cognate proteins in the normal serum, which is derived from a different species from the primary Ab?

Sorry if this question seemed trivial. Thanks in advance.

You're probably right. Still, blocking non-specific binding is not always an exact science. If normal serum works, great! But, as you implied, we won't be surprised if it does not work as expected.