Antibodies to species homologous tissue antigens in the rat. I. Naturally occurring circulating antibodies.

One hundred and four sera of rats were examined for antibodies reacting in the complement-fixation test with species-homologous tissue antigens. Antibodies were found in twenty-three out of thirty-eight sera of untreated animals and in twenty-nine out of sixty-six sera of Freund's adjuvant-treated animals, the highest titre being 1:640. Ten per cent or less of the sera contained antibodies reacting with the homogenate of whole kidney, heart or spleen and with the renal heavy particulate material or mitochondrial fraction. In about 20% of the sera there were antibodies reacting with the renal soluble fraction and in over 40% with the renal microsomal fraction. There was no statistically significant difference between the overall incidence of circulating antibodies in untreated and adjuvant treated rats. The stimulus responsible for the formation of these antibodies to specieshomologous tissue antigens is unknown. There is no evidence that they are of pathogenic significance.