Abstract
It is well known that the human female is more resistant to syphilitic infection than the male, especially during the child bearing period of life. This difference has been attributed to the influence of pregnancy. 1 , 2 Corroborating the observations on human individuals, there is experimental evidence that the pregnant rabbit inoculated at the time of conception reacts to infection less severely than either the normal female or male rabbit. 3 A similar sex variation has been reported in normal rabbits. 4
The essential nature of the forces responsible for this immunologic dissimilarity of the sexes is not known. In seeking an explanation, it is reasonable to assume that some endocrine function peculiar to the female and closely related to reproduction may play a dominant rôle in the protective mechanism of this sex. In this connection it has been shown that the reaction of the rabbit to syphilis depends in part upon the integrity and balance of the glands of internal secretion. 5
One of the most striking phenomena of pregnancy related to the activity of the glands of internal secretion is the elaboration of an estrogenic substance, or substances, which as pregnancy progresses can be demonstrated in large amounts in the circulating blood 6 and in the urine. 7 As it is known that certain secondary sex characters of the female depend upon the presence of this substance, it has occurred to us that the resistance of the female to syphilitic disease may be an analogous character of sex dependent upon the same or similar factors.
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