Pre-weaning mortality in domestic animals results in a significant loss in animal production. Immunizing pregnant dams before parturition is practiced in order to prevent or decrease losses from various threatening diseases. The transferal of maternal immunity to offspring depends on the type of animal species and may occur via the placenta and colostrum or only through colostrum. Maternal, passive immunity of sucklings lasts for several weeks only and decreases in time, therefore young animals must be vaccinated in order to be protected against infectious diseases. Maternal antibodies present in suckling block antigenic epitopes of the vaccine antigens and interfere with their efficacy and, thus, the appropriate time for vaccinating is not easy to determine. It should occur early in the process of maturation, yet not when the level of maternal antibodies is very high in order to facilitate minimizing the immunological window (also know as the window of susceptibility) which exists between decreased maternal immunity and the inefficacy of post-vaccine protection. Maternal antibodies do not interfere with the activity of cellular immunity. In the case of humeral immunity the priming of B lymphocytes, which gain immunological memory, occurs despite the blocking of epitopes and so the ensuing vaccine leads to a more intensive production of antibodies and a higher activity of T lymphocytes. Additionally, the appropriate choice of adjuvant in inactivated vaccines is crucial for the immunogenic effect of early vaccination in offspring.
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