Antiserum, or more colloquially serum, is a preparation of antibodies in blood serum used in the treatment of an infection.  It is derived from the blood of an individual who has survived the disease.  

Although the antiserum does not attack the infection directly, it does bind to the infectious agent which alerts the recipient's immune system, provoking an immediate immune response.  

See also Antitoxin and Anti-venom, which work on the same principle.

Antiserum at Wikipedia

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