Passive immunity refers to short-term protection from disease resulting from transfer of ready-made antibodies (immunoglobulins). This is in contrast to active immunity, which refers to immunologic defense by an animal's own immune system.
Passive immunity may be acquired naturally (eg, maternally derived antibodies transferred to neonates via the placenta or colostrum) or artificially (eg, via donor serum or monoclonal antibodies generated through the use of cloned cell lines).
Polyclonal Antibodies in Animals
Antibodies from one animal, such as those produced in response to vaccination, can be transferred to another. Serum from a donor animal can be administered to susceptible animals to confer immediate but short-lived protection.
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. They can also be used in the treatment of snake envenomation. These immune globulins are generally produced in young horses by a series of inoculations.
Monoclonal Antibodies in Animals
In a normal immune response, antibodies are produced by diverse 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 can be generated through the use of cloned cell lines called hybridomas; these monoclonal antibodies represent an alternative source of passive immunity.
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-IL-31 antibody may be used to block severe itch in dogs with atopic dermatitis.
Monoclonal antibody use has increased greatly in humans. These antibodies are mainly directed against inflammatory cytokines such as tumor necrosis factor and IL-1. They are primarily used for the control of chronic inflammatory diseases, such as rheumatoid arthritis and systemic lupus.
Monoclonal antibodies are also used to combat many human cancers either by attacking cancer cells directly or by enhancing cytotoxic T-cell responses. Caninized monoclonal antibodies are being increasingly used against cancer in dogs.
