Antisera have long been the major reagent in immunology, and this field was for years known as serology. An antiserum is the serum of an animal that has been immunized against an antigen, or more commonly hyperimmunized, to get the highest possible titer of the desired antibodies. Very clearly, vaccination and serotherapy were the first great miracle in fighting against pathogens, long before the discovery of antibiotics. From a medical standpoint, antisera were used to fight quickly against a pathogen or a toxin, because vaccination takes a long time to generate a host immune response. There are several concerns regarding the use of antisera for therapy in humans or in animals. First, one has to be certain that protection is indeed ensured by antibodies and not exclusively by cell-mediated immunity. This is not a trivial matter, because there are only a limited number of instances in which protection is due primarily to antibodies. Second, the use of antisera from a foreign animal species (heterologous antisera) will induce a strong immunization of the host against all the injected antigens. This results in the appearance, within a week or two, of a local and generalized syndrome, called serum sickness, that may include urticaria, local edema, rashes, arthralgia, fever, lymphadenopathy, and, ultimately, severe glomerulonephritis resulting from the formation of immune complexes. This was observed when serotherapy with horse serum was extensively used in humans, especially against diphtheria or tetanus. Diphtheria, a bacterial disease, can now be cured with antibiotics, and children undergo a regular schedule of vaccination that actively prevents the disease. Tetanus remains a very severe disease, with an elevated rate of mortality. Serotherapy is still used because antibodies are extremely effective against the toxin; nevertheless, everyone should be revaccinated every 10 years, because this provides the best and safest protection.
Heterologous antisera should be used only in very exceptional occasions—for example, after bites from highly dangerous snakes. Heterologous antisera are being systematically replaced, when appropriate, by purified immunoglobulins, preferentially of human origin. Intravenous immunoglobulins (IVIgs) are used to correct severe immunodeficiencies of the B-cell compartment. For example, this is the case with X-linked agammaglobulinemia (Bruton disease), which is transmitted genetically through the mother and occurs in young boys. This disease is due to a variety of mutations of the BTK gene (for Bruton tyrosine kinase) that result in an early blockage of B-cell differentiation. IVIgs are also used to help patients with chronic or transient hypoglobulinemia. Whenever the deficiency is severe and would necessitate a lifelong treatment, an allogenic bone marrow graft is performed, which restores the immune system of the patient. IVIgs are also used in therapy of autoimmune diseases, although the mechanisms are not absolutely understood, but may be due in part to a reequilibration of the idiotype network. Specific purified antibody of human origin can be prepared in some selected cases. The most popular preparation is the anti-D (Rhesus) immunoglobulins that are used worldwide for prevention of the hemolytic disease of the newborn (see Alloantibody, Alloantigen).
With the start of organ transplantations, many attempts were made to prevent rejection by blocking the immune system of the recipient. Antilymphocyte serum, prepared in rabbits or in horses, has been used but had the usual limitation linked to the induction of serum sickness mentioned above. It could, however, help for a transient difficult acute period of rejection. It has been replaced by a monoclonal antibody, prepared in the mouse and directed against the CD3 (signaling module) of the T-cell receptor. Used for a short period of time, it proved efficient in down-modulating the immune response of the host, with limited risk of immunization against the heterologous immunoglobulin. Genetic engineering has also been proposed to minimize immunization against heterologous determinants. One classical approach is to insert the six specific hypervariable regions of a murine monoclonal antibody in place of the corresponding regions of a human antibody framework, using protein engineering. This engineered antibody is certainly less immunogenic, but it still is, because of the idiotype determinants that cannot be avoided. Fully engineered human monoclonal antibodies would certainly be ideal for human use. This goal is being worked on actively by pharmaceutical companies. Its is not yet practical, and no one has yet succeeded in making stable "natural" human monoclonal antibodies.
Besides the roles in human therapy mentioned above, antisera remain of course reagents of choice for experimental purposes. They may be used to identify new molecules and to purify them with techniques like immunoaffinity chromatography or immunoprecipitation, which are powerful tools to isolate rare components, such as membrane proteins, receptors, hormones, or diverse ligands. Monoclonal antibodies have tended to replace antisera, which is partly unfortunate and sometimes a mistake, because conventional antisera can be exquisitely specific and are very efficient, potent tools, with a mosaic of specificities that may of great help to the molecular biologist.
