The key point of IBDV immune-complex vaccines is to ensure the correct coating of the vaccine virus with the specific IBDV antibodies (IgY).
This correct coating of the virus is what will prevent the neutralization of the vaccine virus by maternally derived antibodies (MDAs).
The immune complex (virus+IgY) will then be retained in the follicular dendritic cells of lymphoid organs (and probably on macrophages) which will lead to a continuous release of the virus without antibodies. If the protection provided by the mother is still active, the virus released will be neutralized by the MDAs and will not start to replicate.
Once this maternal protection starts to decrease, the virus released will not be neutralized and will start to replicate in the bursa, which means that the bird will automatically be protected by competitive exclusion and the onset of immunity will start.
This onset of immunity depending on the protection needs of each individual chick is known as “Gumboro intelligent vaccination”.
1Whitfill et al. 1995. Avian Diseases 39, (4), 687-699.
2Gelb et al. 2016. Avian Diseases 60 (3), 603-612.
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