An antitoxin is an antibody with the ability to neutralize a specific toxin. Antitoxins are produced by certain animals, plants, and bacteria. Although they are most effective in neutralizing toxins, they can kill bacteria and other microorganisms. Antitoxins are made within organisms, but can be injected into other organisms, including humans. This procedure involves injecting an animal with a safe amount of a particular toxin. Then, the animal’s body makes the antitoxin needed to neutralize the toxin. Later, the blood is withdrawn from the animal. When the antitoxin is obtained from the blood, it is purified and injected into a human or other animal, inducing passive immunity. To prevent serum sickness, it is often best to use antitoxin generated from the same species (e.g. use human antitoxin to treat humans).
History of antitoxin
Antitoxins to diphtheria and tetanus toxins were produced by Emil Adolf von Behring and his colleagues from 1890 onwards. The use of diphtheria antitoxin for the treatment of diphtheria was regarded by The Lancet as the "most important advance of the [19th] Century in the medical treatment of acute infectious disease".
In 1888 Behring was sent to Berlin for a brief service at the Academy for Military Medicine. In 1889, he joined the Institute for Hygiene of the University of Berlin, then presided over by Robert Koch. Between 1889 and 1895 Behring developed his pioneering ideas on serum therapy and his theory of antitoxins.
Early 1887, in Bonn, Behring had found that the serum of tetanus-immune white rats contained a substance that neutralized anthrax bacilli. He saw this as the cause of “resistance.” 
On 4 December 1890 Behring and Kitasato Shibasaburō published their first paper on blood-serum therapy. On 11 December another report, signed by Behring, discussed the blood-serum therapy not only in the treatment of tetanus, but also of diphtheria.
- (Report) (1896). "Report of the Lancet special commission on the relative strengths of diphtheria antitoxic antiserums". Lancet. 148 (3803): 182–95. doi:10.1016/s0140-6736(01)72399-9.
- Dolman, C.E. (1973). "Landmarks and pioneers in the control of diphtheria". Can. J. Public Health. 64 (4): 317–36. PMID 4581249.
- "Emil von Behring Facts, information, pictures | Encyclopedia.com articles about Emil von Behring". www.encyclopedia.com. Retrieved 2016-05-17.
- Cushing, H. B. Results of the Use of Scarlet Fever Antitoxin *. Canadian Medical Association Journal. August, 1926, pg. 936.
- Zingher, Abraham . THE DICK TEST AND ACTIVE IMMUNIZATION WITH SCARLET FEVER STREPTOCOCCUS TOXIN *. American Journal of Public Health . November, 1924, pg. 955.
- Antitoxins at the US National Library of Medicine Medical Subject Headings (MeSH)