1. Particles derived from filtrates of organ suspensions by high speed centrifugation were serologically active as shown by agglutination and complement fixation techniques. Particles from brain, liver, lung, kidney, heart muscle, spleen, testicle, and pancreas of various species have been studied.

2. All particles showed a certain degree of organ specificity with the exception of pancreas. Cross-reactions occurred between the particles from various organs from one species, which were more marked when complement fixation technique was employed than by the agglutination test. However, agglutination always appeared earlier and was stronger, and complement fixation was positive in higher dilutions of antigen in the presence of homologous antiserum than with heterologous antisera.

3. The cross-reactions did not depend on the occasional precipitins for serum and the agglutinins for the red cells of the species from which the particles were derived, nor did they bear a relation to Wassermann and Forssman antibodies present in some of the sera.

4. The organ specific differentiation of the particles from various organs could more clearly be demonstrated by two means: The antiserum could be diluted in such a way that only the homologous reaction still showed a positive result while the cross-reactions had become negative; or the cross-reacting antibodies could be absorbed by heterologous particles and the homologous reaction was still more or less intact.

5. In addition to the organ specific differentiation, most particles were found to exhibit species specificity. While the particles derived from kidney, lung, testicle, and heart muscle aggregated only in the presence of the antiserum against the corresponding organ particles from the homologous species, brain particles reacted with brain antisera against both homologous and heterologous species alike. Absorption of brain particle antisera with brain preparations from a heterologous species removed all antibodies. Liver particle preparations showed an intermediate position in that all liver preparations with the exception of rabbit liver particles were aggregated by any liver particle antiserum. However, absorption with liver particles from a heterologous species left a distinct species specific reaction in the serum.

6. The antigens involved are all destroyed by heating to 100° C. for a few minutes with the exception of brain particles, which after 20 minutes at 100° C. still gave complement fixation almost to the same strength as the untreated controls.

7. Alcoholic and ether extracts of brain reacted with the brain particle antisera only. All alcoholic or ether extracts of other organs gave no complement fixation. None of the various other organ particle antisera tested contained antibodies for these extracts.

8. The relationship between the heat-stable and the alcohol-soluble brain particle antigen studied by absorption technique revealed that there were two antigens present, both organ specific and independent of the species, the one alcohol- and ether-soluble, the other not soluble in these solvents but heat stable. Some of the sera showed besides a few species specific antibodies.

9. Preliminary evidence has been gathered to show that no iso-immunization could be obtained with any one of the organ particles. As far as cytotoxic activity of the sera is concerned only the kidney particle antisera have been studied for nephrotoxins; these failed to reveal any such activity in the mouse.

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