Passive immunization involves the production of antibodies in one animal by active immunization. These antibodies can be stored (as immunoglobulins) and then 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. Immunoglobulins 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. These immunoglobulins are generally produced in young horses by a series of immunizing inoculations.
To check the potency of preparations of immunoglobulin, comparison is made with an international biological standard and expressed in international units (IUs). Tetanus immunoglobulin is given to animals to confer immediate protection against tetanus. At least 1,500–3,000 IUs of immunoglobulin should be given to horses and cattle; at least 500 IUs to calves, sheep, goats, and pigs; and at least 250 IUs to dogs. The exact amount varies with the amount of tissue damage, degree of wound contamination, and time elapsed since injury. Tetanus immunoglobulin is of little use once clinical signs appear, although massive doses of up to 300,000 IUs 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 antigen, they are a heterogeneous mixture of proteins. Homogeneous antibodies can be generated through the use of cloned cell lines called hybridomas; these monoclonal antibodies represent an alternative source of passive immunization. Currently, however, these are mainly made by mouse hybridomas (and thus consist of mouse antibodies) and may sensitize other animal species.
Monoclonal antibodies are commonly used in diagnostic tests. Because they are homogeneous and specific, these antibodies can differentiate between closely related infectious agents in a manner impossible with conventional antibodies. For example, they can differentiate between the rabies viruses obtained from skunks, bats, or dogs.