Further Studies of Xh, a Serum Protein Antigen in Man 1

Abstract

Xh has been identified as a serum protein antigen whose expression is both age and sex dependent, and is shown here to be identical with the antigen previously reported as Pa 1. On immunodiffusion, approximately 75% of randomly selected sera from females of all ages form a precipitin line with specific rabbit antiserum. Frequency of the antigen in females is correlated with age; 94% over 19 years of age are positive while only 55% under 19 are positive. Approximately 47% of male sera are positive; no significant changes in frequency associated with age of males were found. The mean level of the antigen in the sera of females over 19 years of age is approximately twice that in male sera. The heritability of Xh expression on the basis of a study of male twins was 0.66. A study of 88 families, with 181 offspring selected so that female offspring were at least 20 years old, was inconclusive in two ways: (1) two female offspring exceptional to the hypothesis of sex linkage were found and (2) no negative by negative matings were observed. Thus, although the Xh data are suggestive of a genetic polymorphic system, the influence of physiological factors on Xh expression and the findings, reported here, of similarly high frequencies in adult females of all racial groups, make it more probable that this antigen is an isotypic marker. Reactivity with anti-Xh was found among sera of some of the great apes and Old and New World monkeys but not among prosimians and lesser mammals. The isotypic specificity is presumed to have arisen during the speciation process leading to the present day anthropoid apes and man.