Vaccines for counteracting inhibition of neutrophil degranulation by pathogenic cellular microorganisms comprise one or more purines conjugated to antigenic carriers. The purines include adenine and/or guanine in the form of a base, nucleoside, or nucleotide. The vaccines can be used with cattle to increase resistance to infections by Brucella abortus (brucellosis) and/or Haemophilus somnus (thromboembolic meningoencephalitis) and related symptomatic complex.