The link below can be useful to get an answer for your question...

http://www.pierce-antibodies.com/targets/t/MouseSecondaryAntibodies.cfm

What they mean is that in mouse blood there are mouse antibodies. If you then use a anti-mouse secondary antibody for western blotting, your secondary antibody will recognize the heavy chain (50kDa) and light chain (25kDa) of these mouse blood antibodies, so you will have two extra bands on your blot in addition to the 6 that the cocktail intends to give you. Because the heavy chain and light chain bands are denatured proteins (SDS, reducing agent, boiling), they do not have the native conformation that the primary antibodies you use for western blot will have. They propose that you use an anti-mouse secondary antibody that recognizes the non-denatured structure of the mouse primary antibody. The most widely-used secondary antibody with these characteristics is the TrueBlot secondary from Rockland. In general people use these for immunoprecipitation followed by western blot, again because they do not want to see the heavy chain and light chain bands on the western. Here is a technical article about the subject: http://www.tebu-bio.com/index.php?module=tech-info&id_cms=878&type_cms=3

Thanks a lot to both of you!!

Mark, your answer has been really useful!

Thank you! :-)