Page 776 - Veterinary Immunology, 10th Edition

VetBooks.ir Active Immunization

Active immunization has several major advantages over passive
immunization. These include the prolonged period of protection

and the recall and boosting of this protective response by repeated
injections of antigen or by exposure to infection. An ideal vaccine
for active immunization should therefore give prolonged strong
immunity. This immunity should be conferred on both the animal
immunized and any fetus carried by it. In obtaining this strong

immunity, the vaccine should be free of adverse side effects. (In
effect, it should stimulate adaptive immunity without triggering the
inflammation associated with innate immunity.) The ideal vaccine

should be cheap, stable, and adaptable to mass vaccination; ideally,
it should stimulate an immune response distinguishable from that
due to natural infection so that immunization and eradication may
proceed simultaneously.
In addition to these requirements, effective vaccines must have

other critical properties. First, antigen must be delivered efficiently
so that antigen-presenting cells can process antigen and release
appropriate cytokines. Second, both T and B cells must be

stimulated so that they generate large numbers of memory cells so
that protection will last for as long as possible. Third, helper and
effector T cells must be generated to several epitopes in the vaccine
so that individual variations in MHC class II polymorphism and
epitope properties are minimized. Fourth, the immune response

triggered by the vaccine must be appropriate to the infectious
agent, in other words, antibodies or cell-mediated immunity as
appropriate.

Living and Killed Vaccines

Unfortunately, two of the prerequisites of an ideal vaccine, high
antigenicity and absence of adverse side effects, are sometimes
incompatible. Modified live viruses infect host cells and undergo

replication. The infected cells then process endogenous antigen. In
this way, live viruses trigger a response dominated by CD8 +
cytotoxic T cells, a type 1 response. This may be hazardous because

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