MSD Manual

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Professional Version

Active Immunity in Animals


Ian Rodney Tizard

, BVMS, BSc, PhD, DSc (Hons), DACVM, Department of Veterinary Pathobiology, College of Veterinary and Biomedical Sciences, Texas A&M University

Reviewed/Revised Oct 2023

Active immunity can be acquired through natural exposure or can be vaccine induced.

Vaccine-induced immunity involves administration of vaccines containing antigenic molecules (or nucleic acids encoding these molecules) derived from infectious agents. In response, vaccinated animals generate large numbers of memory cells, and develop rapid, specific, and robust adaptive immune responses that provide long-lasting protection against disease (albeit not necessarily infection) due to those agents.

Multiple factors determine whether a vaccine can or should be used. First, the actual cause of the disease must be determined. For example, although Mannheimia haemolytica can be isolated consistently from the lungs of cattle with respiratory disease, these bacteria are not the sole cause of this syndrome, and vaccines against the primary viral pathogens are required for full protection.

An ideal vaccine has several characteristics:

  • Vaccination should rapidly confer prolonged, strong immunity in vaccinated animals.

  • Depending on the nature of the pathogen, vaccination should induce the most protective response (eg, dominated by cytotoxic T cells vs antibodies, categorized as type 1 and type 2 responses, respectively).

  • Vaccination should stimulate responses distinguishable from those due to natural infection so that vaccination and eradication may proceed simultaneously.

Vaccination is not always an innocuous procedure; adverse effects can and do occur. Therefore, all vaccinations must be governed by the principle of informed consent. The risks of vaccination must not exceed those caused by the disease itself.

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