-------------------+-----------------------+ | Serum | +-----------------------+ | I | II | III | IV | ------------+------+-----+-----+-----+-----+ Corpuscles | I | - | + | + | + | +------+-----+-----+-----+-----+ | II | - | - | + | + | +------+-----+-----+-----+-----+ | III | - | + | - | + | +------+-----+-----+-----+-----+ | IV | - | - | - | - | ------------+------+-----+-----+-----+-----+ The active principle in the serum is called “agglutinin” or “hæmolysin,” according to the degree of the reaction, and the corpuscles are rendered sensitive to this by the possession of an “iso-agglutinin” or “iso-hæmolysin.” Sometimes the corpuscles are said to have “agglutinophilic” properties. It may be stated, therefore, that the serum of Group I entirely lacks agglutinins, whereas the corpuscles of Group IV lack iso-agglutinins. All these terms, like the “amboceptors,” “receptors,” and “haptophores” of Ehrlich, are used to conceal ignorance rather than as an expression of knowledge, but, until more light has been shed upon the nature of the reactions, ignorance must be abbreviated. It is now clear that the blood as a whole contains two sets of reactions which are independent. These properties reside in the serum and in the corpuscles respectively, and the reactions are complementary between Groups II and III, that is to say, the serum of each group agglutinates the corpuscles of the other. It will be seen from the table that the serum of Group I blood does not agglutinate the corpuscles of any of the other groups, and conversely the corpuscles of Group IV are not agglutinated by the serum of any of the other groups. Individuals of Groups I and IV have therefore been named “universal recipients” and “universal donors” respectively. This implies that if the recipient be found to belong to Group I, the blood of any donor may be transfused into his veins irrespective of his group, and that if the donor be of Group IV, his blood may be used for transfusion irrespective of the group of the recipient. These statements may be accepted as true in an emergency, but important reservations may have to be made under certain conditions.