Antiserum is a product derived from blood which can be used to activate the immune system of someone who has been exposed to a pathogen or toxin so that the immune system can eliminate it. Antisera are used when other treatments are not available, or as one line of defense in a treatment plan, depending on the specifics of the situation. Numerous companies manufacture antisera for medical and research use, and the most common application of antisera is as antivenin products used to treat exposure to poisonous snakes and other poisonous animals.
The antiserum consists of blood serum, a form of purified blood plasma which is loaded with polyclonal antibodies produced by the host organism. Polyclonal antibodies are clones of parent cells which have produced antibodies to one or more antigens. When the antibodies enter the body of the patient, they latch on to the antigens they recognize and the immune system sees them so that it can attack. Essentially, the antiserum acts like a flag, latching on to antigens and tagging them so that they can be seen by the immune system.
Several toxins and pathogens rely on attacking while the immune system is left dormant. They are comparatively weak, and when the immune system wakes up with an infusion of antiserum, it can eliminate the hostile invaders. Sources for antiserum vary, depending on what kind of pathogen or toxin someone has been exposed to.
One source is a human being who managed to survive the infection or venomous attack. For example, with Ebola, a dangerous disease which resists all other forms of treatment, antisera purified from the blood of a few lucky survivors has been used to successfully treat people who have been exposed to the disease. Antibodies derived from human survivors are sometimes used in the early stages of epidemics, while researchers are still identifying the disease and developing an approach to treatment.
Commercially, antisera can be produced from animals which carry the infection, but don't get sick, or from animals which are exposed to small quantities of the pathogen over time. These exposures give the immune system of the animal time to respond and develop antibodies, and blood can be drawn and processed to develop an antiserum. Antisera can be short lived, and may be quite costly because of the amount of work involved in production, so they are usually kept stocked in major medical centers only, with smaller hospitals and clinics making requests when they need specific antisera.