Passive immunization involves the production of antibodies in one animal by active immunization and transfer to another. The donor animal can be bled and its serum administered to susceptible animals to confer immediate but short-lived protection. The transfer of maternal antibody to offspring via the placenta or colostrum is the natural (and very important) form of passive immunization. Immune globulins may be produced in cattle against anthrax, in dogs against distemper virus, and in cats against panleukopenia virus. Their most important role is in protection against toxigenic organisms, such as tetanus, botulism, or Clostridium perfringens. It can also be used in the treatment of snake envenomation. These immune globulins are generally produced in young horses by a series of immunizing inoculations.
Tetanus immune globulin is given to animals to confer immediate protection against tetanus. At least 1,500–3,000 IU of immune globulin should be given to horses and cattle; at least 500 IU to calves, sheep, goats, and pigs; and at least 250 IU to dogs. The exact amount varies with the amount of tissue damage, degree of wound contamination, and time elapsed since injury. Tetanus immune globulin is of little use once clinical signs appear, although massive doses of up to 300,000 IU may help.
In a normal immune response, antibodies are produced by many different plasma cell populations and are thus said to be polyclonal. Although these antibodies all combine with a specific organism, they are a heterogeneous mixture of proteins that react to different epitopes of that organism. Homogeneous antibodies that react to a single epitope now can be generated through the use of cloned cell lines called hybridomas; these monoclonal antibodies represent an alternative source of passive immunization. Whereas the earliest monoclonal antibodies were made by mouse hybridomas (and thus consist of mouse antibodies), molecular engineering techniques now permit them to be altered to match the recipient species. For example a caninized monoclonal anti-interleukin-31 may be used to block itching dogs with atopic dermatitis.