The J substance of cattle appears to be a constituent of the serum, from which it may be acquired by the cells so that the cells are lysed by anti-J serum. The J substance of the serum will inhibit the interaction between cells carrying J and anti-J sera. The amount of J substance present on the cells or in the serum of an individual is remarkably constant, and the cellular reactivity presumably depends on the concentration of soluble J in the serum. Cattle blood may be divided into three groups: (1) JCB, those with J substance on the cells and in the serum; (2) Js, those with J substance only in the serum, and (3) ja, those without J substance but whose sera may contain anti-J. The inheritance of these three classes may be explained by assuming a triple allelic series of causative genes—Jcz, Js and ja, of descending order of dominance.

The J substance is present in the serum of the newborn calf in the same, or nearly the same, concentration that will obtain in later life. It is not detectable on the red cells of the newborn calf, but is gradually taken up by the cells during the first weeks of life. Calves with the J substance in their sera which have ingested colostrum with anti-J have exhibited no untoward effects. Seemingly, the antibodies to J which are received by the calf are bound to the J substance of the serum, since anti-J can not be detected in the serum of these calves. Other evidence suggesting strongly that the antibodies to J are bound to the J substance of the serum before combining with the red cells is indicated by the lack of adverse reaction when transfusions are made between animals of class Jcs and those possessing anti-J.

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