A method was described for the production of an antileucocytic serum relatively free of hemolytic antibodies.

The parenteral injection of this antiserum into guinea pigs was shown to depress the number of neutrophils in the circulating blood with less effect upon the lymphocytes. The monocytes, eosinophils and basophils reacted much as they did to similar injections of normal rabbit serum. The numbers of platelets and red cells were not significantly altered.

Following intracardial injections of the antiserum the neutrophils disappeared from the blood within 5 minutes and since large numbers of them could not be found in the tissues at this time it is evident that they very rapidly disintegrated beyond the stage of morphological recognition upon contact with the antibody.

Following intraäbdominal injections of the antiserum the characteristic disappearance of neutrophils occurred within one to 7 hours and persisted to between 24 and 48 hours. In 72 to 96 hours the neutrophils returned to normal or to somewhat increased numbers.

When injections were repeated a tolerance developed so that an equal dose caused less effect. It was shown that this tolerance was probably not due to antibodies developed against the proteins of rabbit serum or against the leucotoxic antibodies themselves.

A striking degree of myeloid hyperplasia was present in the bone-marrow of repeatedly injected animals, as well as a moderate degree of lymphoid and endothelial hyperplasia in the lymphnodes and spleen.

There was no constant evidence of damage to other body-cells with the amount of antiserum used.

It was possible to prolong the period of neutropenia to 4 or 5 days by daily injections of increasing amounts of antiserum without seriously affecting the general condition of the animals. It is probable that the period could have been further prolonged by properly increasing the amounts of antiserum injected. It was shown that neutrophils were not available for local tissue-reactions to the subcutaneous injection of virulent staphylococci or to the intraäbdominal injection of a beef broth-gum acacia mixture in animals rendered neutropenic by an injection of the antiserum.

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