For decades, doctors have recom that couples trying to conceive should have intercourse as often as possible, not only during ovulation (obviously) but at other times as well. Doing so is known to improve the odds of success, though the reasons have remained a puzzle. Now two new studies suggest, somewhat radically, that sex alters a woman’s immune system in ways that affect her chances of conceiving.

数十年来，医生会向试图怀孕的伴侣建议，尽可能频繁地性交，不仅是在排卵期（显然），其他时间也一样。大家都知道这样做可以增加成功几率，不过其中的原因却一直是个谜。现在，有两项新研究颇具重大意义地显示出，性活动会改变女性的免疫系统，从而影响她的受孕几率。

The data for the studies come from the Kinsey Institute for Research on Sex, Gender and Reproduction and the Center for the Integrative Study of Animal Behavior, both at Indiana University, where researchers recruited 30 healthy, premenopausal women, all of whom were heterosexual and not trying to become pregnant. About half of them were sexually active, using condoms or intrauterine devices as birth control, while the others were abstinent. The volunteers provided blood, saliva and other samples throughout several menstrual cycles. Apart from their sex lives, women in both groups were broadly similar in terms of health and lifestyle.

这两项研究的数据分别出自金赛性、性别与生殖研究所(Kinsey Institute for Research on Sex, Gender and Reproduction)和动物行为综合研究中心(Center for the Integrative Study of Animal Behavior)。两家机构均设在印第安纳大学(Indiana University)。研究人员招募了30名更年期之前的健康女性，全部为异性恋，而且并未尝试怀孕。其中大约一半人性生活活跃，采用安全套或宫内节育器避孕，另一半人则禁欲。志愿受试人员提供了数个月经周期的血液和唾液等样本。除了性生活方面的差异，两组女性在健康状况和生活方式方面大致相似。

They soon revealed distinctly different immune-system responses over the course of a menstrual cycle. According to one of the new studies, published in September in the journal Fertility and Sterility, the sexually active women displayed heightened levels of a certain immune cell when their reproductive systems were preparing to release an egg but before they were able to become pregnant. Later in the menstrual cycle, when conception was possible, these women developed higher levels of a different type of immune cell — one known to help a body recognize and ignore nonhazardous foreign cells, like those in a fetus. There were no similar changes in the immune systems of the abstinent women.

他们很快发现，在一个月经周期内，两组女性的免疫系统反应截然不同。其中一项发表于《生育和不育》(Fertility and Sterility)杂志9月刊的研究显示，在女性的生殖系统正准备排卵但还无法受孕之前，性生活活跃的女性某类免疫细胞的水平会比较高。在月经周期后期，也就是有可能怀孕时，这些女性体内又有另一种不同类型的免疫细胞的水平会升高——这种免疫细胞有助于人体辨认和忽视无害的外来细胞，比如来自胎儿的细胞。禁欲女性的体内没有类似的免疫系统变化。

The other study, published in Physiology and Behavior, found other immunological differences. Sexually active women early in their cycles developed more antibodies of a type that lives in the mucus lining the reproductive tract and represents a threat to sperm and fetuses. Levels of these antibodies dropped later in the cycle, while the numbers of a different germ-fighting antibody in the blood but not in the reproductive tract grew.

发表在《生理学与行为》(Physiology and Behavior)杂志上的另一项研究发现了一些其他的免疫系统差别。在月经周期早期，性生活活跃的女性体内某种抗体水平比较高。该抗体存在于生殖道粘膜内，对精子和胎儿都构成威胁。在月经周期后期，这种抗体的水平会下降，另一种不同的抗菌抗体的数量则会增多。这种抗体存在于血液里，而不存于生殖道中。

Together, these findings indicate that ‘‘the more frequently a woman engages in sexual activity, the more often her immune system gets the message that it’s time to reproduce,’’ says Tierney Lorenz, the research scientist at the Kinsey Institute who was the lead author of both studies. The research did not examine conception or rates of illness and infection, so the real-life implications of the differences in immunity remain uncertain. It’s also not clear how the immune system knows someone is having sex. There may be messages from the brain or extra changes in hormones, Lorenz says. Sexual partners may even exchange elements of their microbiome, prompting changes in each other’s immunity. For now, the primary lesson Lorenz takes away from these studies, she says, is ‘‘awe for the elegance of our flexible, powerful, socially aware immune system.’’

这些发现共同显示出，“女性性生活越频繁，她的免疫系统就会更频繁地接到信号，提示是进行繁殖的时候，”在金赛性、性别与生殖研究所做研究的科学家、上述两项研究的第一作者蒂尔尼·洛伦茨(Tierney Lorenz)说。这项研究没有调查受孕情况，也没有研究患病和感染的几率，所以不同的免疫状况在真实生活中会有什么影响仍未可知。同样还不清楚的是，免疫系统如何知道某人在做爱。洛伦茨表示，有可能是大脑发送了信号，或者有荷尔蒙的额外变化进行提示。性伴侣甚至可能会交换体内的菌群，由此引发对方体内的免疫系统发生变化。洛伦茨表示，她目前从这两项研究中得到的最大收获，是“对我们那灵活、强大和具备外界感知力的免疫系统的精妙，产生了敬畏之心。”

This column appears in the October 25, 2015 issue of The New York Times Magazine

原文将刊登在2015年10月25日出版的一期《纽约时报杂志》上。