The official unit for tetanus antitoxin is somewhat different, since it is standardized against a _standard toxin_ which is likewise kept at the Hygienic Laboratory. The unit is defined as “ten times the amount of antitoxin necessary to protect a 350 g. guinea-pig for 96 hours against the _standard test dose_” of the standard toxin. The standard test dose is 100 M. L. D. of toxin for a 350 g. guinea-pig. To express it another way, one could say that a “unit” of tetanus antitoxin would protect one thousand 350 g. guinea-pigs from 1 M. L. D. each of standard tetanus toxin. Various methods have been devised for increasing the amount of antitoxin in 1 cc. of solution by precipitating out portions of the blood-serum proteins and at the same time concentrating the antitoxin in smaller volume. It is not considered necessary in a work of this character to enter into these details nor to discuss the process of standardizing antitoxin so that the exact amount of “units” per cc. may be known. CHAPTER XXVIII. RECEPTORS OF THE SECOND ORDER. AGGLUTININS. Charrin and Rogers appear to have been the first (1889) to observe the clumping together of bacteria (_Pseudomonas pyocyanea_) when mixed with the blood serum of an animal immunized against them. Gruber and Durham (1896) first used the term “agglutination” in this connection and called the substance in the blood-serum “agglutinin.” Widal (1896) showed the importance of the reaction for diagnosis by testing the blood serum of an infected person against a known culture (typhoid fever). It is now a well-known phenomenon that the proper injection of cells of any kind foreign to a given animal will lead to the accumulation in the animal’s blood of substances which will cause a clumping together of the cells used when suspended in a suitable liquid. The cells settle out of such suspension much more rapidly than they would otherwise do. This clumping is spoken of as “agglutination” and the substances produced in the animal are called “agglutinins.” If blood cells are injected then “hemagglutinins” result: if bacterial cells “bacterial agglutinins” for the particular organism used as “glanders agglutinin” for _Pfeifferella mallei_, “abortion agglutinin” for _Bacterium abortus_, “typhoid agglutinin” for _Bacterium typhosum_, etc. The phenomenon may be observed either under the microscope or in small test-tubes, that is, either _microscopically_ or _macroscopically_.