Just keep in mind....
A dog can have immunity (humoral/antibody ability), but have a negative titer.
A dog can have a positive titer, and NOT have humoral immunity (meaning, they only have cell-mediated immunity).
Titers aren't a definitive answer...altho they can be helpful.
This explains it so much better:
http://www.vet.bc.ca/site/view/54231_Titers.pml