Passive immunization

Passive immunization involves the production of antibodies in one animal by active immunization, followed by transfer of these antibodies to susceptible animals to confer immediate protection. The transfer of maternal antibody to offspring via the placenta or colostrum is the natural (and very important) form of passive immunization. Antisera may be produced in cattle against anthrax, in dogs against distemper, and in cats against panleukopenia. Their most important role is in protection against toxigenic organisms, eg, Clostridium tetani or C perfringens . Such antisera are known as immune globulins and are generally produced in young horses by a series of immunizing inoculations.

To check the potency of preparations of immune globulin, comparison is made with an international biological standard. An International Unit (IU) of tetanus immune globulin is the specific neutralizing activity contained in 0.03384 mg of the international standard. The USA Standard Unit (AU) is twice the IU.

Tetanus immune globulin (tetanus antitoxin) 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.

To reduce their antigenicity for other species, immune globulins of equine origin are usually treated with pepsin.

Monoclonal Antibodies:

Antibodies produced by the normal immune response are derived from many different plasma cells. As a result, although they combine with a specific antigen, they are a heterogeneous mixture. Homogeneous antibodies can be generated through the use of cell lines called hybridomas; these, called monoclonal antibodies, represent an alternative source of passive protection. Currently, however, these are mainly made by mouse hybridomas (and thus consist of mouse antibodies) and may sensitize other animal species. Nevertheless, mouse monoclonal antibodies against the K99 pilus antigens of E coli can be given PO to calves to protect them against diarrhea caused by this organism, although this product is no longer commercially available. Monoclonal antibodies also are used in diagnosis. Because they are homogeneous and specific, they have the ability to differentiate between closely related infectious agents in a manner that is impossible with conventional antibodies. For example, they can differentiate between the rabies viruses obtained from skunks, bats, or dogs.