Can primary antibodies be re-used?

It can be tempting to re-use a primary antibody solution. However, do not be surprised if there is no signal at all, when doing so. All of the primary antibody binds in the first incubation, leaving nothing for the second experiment. In such cases, the primary antibody is often blamed for the failure, but the fact is that the antibody is just not present in the solution to do the job. Re-using primary antibodies is also not recommended in case of quantitative work. 

How much specific antibody can be found in serum?
How to get an intense signal, using a weak antibody?