Antitoxin: An antibody capable of destroying microorganisms including viruses and bacteria.

An antitoxin provides passive immunity. For example, if a child gets whooping cough (diphtheria), an antitoxin prepared in horses against diphtheria may be useful in treatment. The antitoxin can only be of short-term value because the antibodies against diphtheria were made by the horse and the child is just the passive recipient of the antibodies.

The antitoxin is directed against a toxin, one of a number of poisons produced by certain plants, animals, and bacteria.

The term "toxin" is frequently used to refer specifically to a particular protein produced by some higher plants, animals and pathogenic (disease-causing) bacteria. A toxin typically has a high molecular weight (as compared to a simple chemical poison), is antigenic (elicits an antibody response), and is highly poisonous to living creatures.

The word "toxin" comes from the Greek "toxikon" = arrow poison.