Before the operation of blood transfusion is carried out it is necessary to test the compatibility of the prospective donor's blood with that of the patient; that is, it must be determined whether the blood corpuscles of the donor will remain intact in the circulation of the patient. It is altogether probable that no two human individuals possess blood having exactly the same chemical composition; indeed, the possible different combinations of demonstrated different substances in the corpuscles alone have been estimated to be about 4000 (1).

Fortunately, however, the differences in the blood plasma and most of those in the corpuscles, although they are, perhaps, the cause of certain unpleasant symptoms, such as a chill followed by a rise of temperature, do not constitute an incompatibility such as could contraindicate the use of the blood of an individual for transfusion.

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