Puppies are protected against many infectious diseases through compounds called antibodies, which they receive in the first few hours from their mother's milk (colostrum). Early vaccination is pointless because these antibodies prevent vaccines working properly. However, by about six weeks the immunity provided by the mother begins to wear off. Therefore, puppies should be given their first vaccination at about 6-8 weeks and then given a second vaccination at about 8 - 10 weeks. The last set of vaccinations should be given around 12 weeks. Once puppies are vaccinated they can mix with other dogs. Early socialisation is beneficially for development. Until your puppy has received all its injections (and for a few days after), it should not mix with other dogs unless you can be certain that they are fully vaccinated and free of disease.