# Why don't female immune systems reject sperm? **URL:** \ **Category:** Factual Questions\ **Created:** [May 16, 2012, 2:43am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055 "2012-05-16T02:43:32Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [May 16, 2012, 2:43am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/1 "2012-05-16T02:43:32Z")
If it’s foreign material, why isn’t it fought off like an infection? ---
**Author:** ![RearEchelon](https://avatars.discourse-cdn.com/v4/letter/r/b4bc9f/32.png) [@RearEchelon](https://boards.straightdope.com/u/RearEchelon)\ **Post date:** [May 16, 2012, 2:45am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/2 "2012-05-16T02:45:28Z")
[Sometimes it is.](http://articles.nydailynews.com/2009-11-05/entertainment/17939193_1_abcnews-com-allergic-seminal) ---
**Author:** ![Babale](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/babale/32/15666_2.png) [@Babale](https://boards.straightdope.com/u/Babale)\ **Post date:** [May 16, 2012, 2:45am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/3 "2012-05-16T02:45:34Z")
Because if a woman’s immune system does that, she wouldn’t pass on her genes (for obvious reasons). ---
**Author:** ![MichaelEmouse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/michaelemouse/32/101_2.png) [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)\ **Post date:** [May 16, 2012, 2:49am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/4 "2012-05-16T02:49:22Z")
The fundamental answer is: Maybe some early mammal female bodies fought sperm like an infection. They had a hard time passing down their genes. Foreign material doesn’t necessarily result in the body fighting it. Saliva is frequently transferred. Everytime you eat and drink, foreign material makes it into you body. Perhaps the question should be: What are the necessary and sufficient causes of the body fighting off foreign material? ---
**Author:** ![oliversarmy](https://avatars.discourse-cdn.com/v4/letter/o/eb8c5e/32.png) [@oliversarmy](https://boards.straightdope.com/u/oliversarmy)\ **Post date:** [May 16, 2012, 2:53am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/5 "2012-05-16T02:53:25Z")
Because men and women are both human. Antibodies are meant to fight infection, not reproduction. That is not snark. Life on earth is built on the power to reproduce. Immune systems are set up to allow reproduction. There may be some instances where the body rejects the basic systems, but in most cases, it does not. Hence, you. ---
**Author:** ![alfabex](https://avatars.discourse-cdn.com/v4/letter/a/3bc359/32.png) [@alfabex](https://boards.straightdope.com/u/alfabex)\ **Post date:** [May 16, 2012, 3:30am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/6 "2012-05-16T03:30:52Z")
I’m having trouble finding a cite, but I remember from high school biology that the vagina actually is a hostile environment for sperm, that’s why there’s millions of them in a single discharge. Only about fifty make it to anywhere near the egg. This doesn’t have to do with the immune system, though. It’s because sperm is a base and vaginal fluids are acidic. ---
**Author:** ![zoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zoid/32/336_2.png) [@zoid](https://boards.straightdope.com/u/zoid)\ **Post date:** [May 16, 2012, 3:37am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/7 "2012-05-16T03:37:11Z")
> [@MichaelEmouse](#): > > Perhaps the question should be: What are the necessary and sufficient causes of the body fighting off foreign material? That’s actually a really good question I’d like the answer to as well. If we don’t get it here will you start a GQ about it? ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [May 16, 2012, 4:22am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/8 "2012-05-16T04:22:34Z")
The female body doesn’t fight sperm because sperm never enter the female body. It’s really that simple. The “interior” of the vagina and uterus are not inside the body, they are just infoldings of the body and are no more inside the body than the armpit is. In simplified terms, the human immune system resides in the bloodstream. there is no blood inside the vagina or uterus, and hence there is nothing to mount an immune response. Asking why the immune system doesn’t reject sperm makes no more sense than asking why the immune system doesn’t reject dental plaque or bellybutton lint. The immune system doesn’t operate on the outside of the body. You can certainly develop an allergy to something on the outside of the body, but the immune systems can’t actually “reject” something just because it is placed on the outside. Interestingly immune rejection of sperm is far more of a problem within the male body. Because sperm only contain half the genetic material of the parent, they have different surface proteins and sugars and if they come into contact with the bloodstream they will be recognised as an intruder and destroyed. The reproductive tract of male mammals goes to some really extreme lengths to make sure that the sperm never contact the bloodstream and even provides the developing sperm with shepherd cells whose sole purpose is to disguise them form the immune system. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 16, 2012, 4:30am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/9 "2012-05-16T04:30:12Z")
> [@](#): > > Because men and women are both human. That’s not the answer, because the immune system _does_ fight off organ transplants, even though they’re also human. And it should be clear that the OP is really asking “how”, not “why”, since the why is obvious. ---
**Author:** ![Dissonance](https://avatars.discourse-cdn.com/v4/letter/d/85f322/32.png) [@Dissonance](https://boards.straightdope.com/u/Dissonance)\ **Post date:** [May 16, 2012, 4:36am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/10 "2012-05-16T04:36:48Z")
> [@Blake](#): > > The “interior” of the vagina and uterus are not inside the body, they are just infoldings of the body and are no more inside the body than the armpit is. Using this definition, the lungs, anus, intestines, mouth and throat aren’t inside the body. Clearly they, as well as the vagina and uterus are. > [@](#): > > In simplified terms, the human immune system resides in the bloodstream. there is no blood inside the vagina or uterus, and hence there is nothing to mount an immune response. [The uterus is supplied by arterial blood both from the uterine artery and the ovarian artery.](http://en.wikipedia.org/wiki/Uterus#Blood_supply)[Vessels of the uterus and its appendages, rear view.](http://upload.wikimedia.org/wikipedia/commons/d/d4/Gray589.png) ---
**Author:** ![Crazyhorse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crazyhorse/32/31_2.png) [@Crazyhorse](https://boards.straightdope.com/u/Crazyhorse)\ **Post date:** [May 16, 2012, 4:45am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/11 "2012-05-16T04:45:46Z")
As to the OP, this is a function of the [innate immune system](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1782397/). > [@](#): > > Mucosal surfaces have evolved to handle potential pathogens against a background of selective physiological functions. In the reproductive tract, the immune system must balance the presence of a resident population of bacteria in the vagina with periodic exposure in the uterus and Fallopian tubes of antigens (bacteria and sperm), as well as the long-term exposure of an allogeneic fetus. Failure of the immune system either to rid the reproductive tract of potential pathogens or to resist attacking allogeneic sperm and fetus significantly compromises procreation, as well as the health of the mother. To meet these diverse challenges, the immune system has evolved to be precisely regulated by the ovarian hormones oestradiol and progesterone, which prepare the reproductive tract for successful fertilization, implantation and pregnancy. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [May 16, 2012, 5:28am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/12 "2012-05-16T05:28:11Z")
> [@Dissonance](#): > > Using this definition, the lungs, anus, intestines, mouth and throat aren’t inside the body. Clearly they, as well as the vagina and uterus are. No, they aren’t. The lungs are a bit of a tricky case, but the digestive tract are all outside the body in every sense. Although we think of them as being “inside” they are just part of the outside that passes through. [The uterus is supplied by arterial blood both from the uterine artery and the ovarian artery.](http://en.wikipedia.org/wiki/Uterus#Blood_supply)[Vessels of the uterus and its appendages, rear view.](http://upload.wikimedia.org/wikipedia/commons/d/d4/Gray589.png) [/QUOTE] Yeah, what’s your point? I don’t think you quite understand that all organs are supplied with blood somehow. That doesn’t mean that there is blood within the lumen of those organs. To help you understand the distinction, think about the lungs, kidneys, eyes or stomach. Those organs have a blood supply, but it’s a serious medical issue of you discover that you have blood \*in \*them. See the difference? ---
**Author:** ![Darth\_Panda](https://avatars.discourse-cdn.com/v4/letter/d/ee7513/32.png) [@Darth\_Panda](https://boards.straightdope.com/u/Darth_Panda)\ **Post date:** [May 16, 2012, 5:32am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/13 "2012-05-16T05:32:58Z")
> [@alfabex](#): > > the vagina actually is a hostile environment Prepare for battle! ---
**Author:** ![drachillix](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@drachillix](https://boards.straightdope.com/u/drachillix)\ **Post date:** [May 16, 2012, 5:43am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/14 "2012-05-16T05:43:33Z")
> [@Darth\_Panda](#): > > Prepare for battle! Oh to experience the heat of battle 😃 ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [May 16, 2012, 5:51am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/15 "2012-05-16T05:51:05Z")
> [@Blake](#): > > In simplified terms, the human immune system resides in the bloodstream. there is no blood inside the vagina or uterus, and hence there is nothing to mount an immune response. That’s oversimplified to the point of just being not true. Cells from the immune system frequently travel outside the blood stream. Various immune cells inhabit mucosal surfaces, including the lumen of the reproductive tract. I’m pretty sure these immune cells are just another one of many hazards that sperm face on the way through the reproductive tract. (My wife studies livestock fertility, so I’ll get an answer from her tomorrow, rather than make something up on my own). ---
**Author:** ![WhyNot](https://avatars.discourse-cdn.com/v4/letter/w/c77e96/32.png) [@WhyNot](https://boards.straightdope.com/u/WhyNot)\ **Post date:** [May 16, 2012, 7:32am UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/16 "2012-05-16T07:32:19Z")
Blake, your answers to biology and evolution question are usually spot-on, but this time I think you’ve simplified it too much and so your answer doesn’t really work. I’m not sure I can do any better, though. There are many kinds of immune cells, some of which recognize invaders, some of which go “running” to tell the bone marrow which invader has been found and “remind” the bone marrow how to make specific antibodies to go tag/kill the invaders (if you’ve had this invader before) or “teach” the bone marrow how to make a new antibody (if you’ve never had this invader before). Some are signal cells which trigger your body to send help in the form of more blood and lymph, and these cause inflammation. Some actually attack and engulf invaders and break them down into little bits and others “suck up” the pieces and take them to your intestine to poop out. Some are like little flaggers that attach to invaders and wave down the engulfers so they can work more effectively. There aren’t “send help!” messengers triggered by sperm. The reason why there aren’t is just as said above - because if there were, you and I wouldn’t be having this discussion. There are some cells that don’t trigger a specific immune response, and sperm is one of them. We evolved from animals which lacked an antibody for sperm, and any accidental mutation which someone is born with that does create a specific antibody against sperm is going to be severely selected against for reproducing. It’s an evolutionary dead end, especially before modern medicine. The general “engulf and break down” and “suck up and carry away the pieces” cells are there, but they take some time to work, and they don’t run well, so the moving sperm avoid them for the time it takes to fertilize an egg. Given enough time and a still target, they do their job (which is why women aren’t constantly full of dead sperm), but they do it pretty quietly and without the need for additional help - ie. no “immune response” specific to sperm. The same cells will engulf and break down anything they can get their cell membranes around. So there \*is \*immune response, in that immune cells do notice and engulf and destroy sperm eventually, but its a slow and generalized immune response, and doesn’t cause inflammation and the icky things (like swelling, heat, pain, redness) that inflammation brings with it. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [May 16, 2012, 12:15pm UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/17 "2012-05-16T12:15:09Z")
The had a show on discovery or science channel about female organism/sex/reproduction that showed that the female body is designed to assist sperm get to their destination, but in a way that makes the journey very hard to weed out the weak ones. In the show it demonstrated through a scannable dye how a orgasm propels sperm upward acting like a pump (and got them pretty far up) and also how sperm are directed to the correct fallopian tube and other such ‘helping’. It also showed how the egg (which would be another form of a foreign body) is also cared for and directed and allowed to burrow into the uterus lining. As to why, it seems like reproduction is such a important function that any helping it really gets reinforced in a evolutionary sense. Also just to add, our bodies have many helpful foreign organisms that are not attacked by the imune system - such as gut bacteria, so just because it’s foreign does not mean that our immune system will fight it, it appears to be much more intelligent then that. ---
**Author:** ![KarlGrenze](https://avatars.discourse-cdn.com/v4/letter/k/dbc845/32.png) [@KarlGrenze](https://boards.straightdope.com/u/KarlGrenze)\ **Post date:** [May 16, 2012, 12:36pm UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/18 "2012-05-16T12:36:45Z")
For the most part, really, sperm is on the surface of an open space. It does not invade tissues or attacks cells or anything else that would require an immediate response. Similarly, your gut bacteria is happily reproducing, moving around in the surface of your mucosal intestines with relatively little “attack” by the immune system. Likewise, there is bacteria all over your skin, and you’re not constantly having skin inflammation. Similarly, most people do not have contact dermatitis. It is not as if the vagina/cervix/uterus is a delicate place. It’s a hollow organ, lined by a tough epithelial to mucosal surface that doesn’t let many things break through. But the hollow part, which is where fertilization occurs, is more or less “empty space”. Note that sperm itself CAN cause a reaction (many cases of orchitis, at least in animals, occur when the seminiferous tubules are ruptured), but they’d have to invade tissue and broken barriers, or produce something similar to contact dermatitis (which doesn’t happen in most people). Happy sperm wriggling along surfaces sends very little “danger” signals to the immune response. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [May 16, 2012, 4:40pm UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/19 "2012-05-16T16:40:37Z")
> [@alfabex](#): > > I’m having trouble finding a cite, but I remember from high school biology that the vagina actually is a hostile environment for sperm, that’s why there’s millions of them in a single discharge. Only about fifty make it to anywhere near the egg. This doesn’t have to do with the immune system, though. It’s because sperm is a base and vaginal fluids are acidic. My lecture notes for Physiology have a bullet stating that prostaglandins “suppress immune response of female.” The text we used (Silverthorn, 5th edition) does not mention this, but it does state that the bulbourethral glands produce buffers that neutralize the usually acidic environment of the vagina. The Wikipedia article on Semen mentions that prostaglandins are involved in suppressing an immune response by the female against the foreign semen. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [May 16, 2012, 5:36pm UTC](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055/20 "2012-05-16T17:36:32Z")
> [@alfabex](#): > > I’m having trouble finding a cite, but I remember from high school biology that the vagina actually is a hostile environment for sperm, that’s why there’s millions of them in a single discharge. Only about fifty make it to anywhere near the egg. This doesn’t have to do with the immune system, though. It’s because sperm is a base and vaginal fluids are acidic. This is the basis for the diaphragm as contraception. The sperm only survive the 2 or 3 days they do, if they are in the uterus (fallopian tubes, actually). The ones trapped outside a diaphragm will succumb to the acidic environment within a few hours, at which point it is safe to remove. The spermicide apparently just stops anything trying to make a run around the edge before it’s too late. But yes, the uterus is separated from the internal bloodstream, so organisms there do not experience the full onslaught of the bloodstream immune system, nor irritate it enough to trigger a major response. the placenta, IIRC, has a special mechanism as part of reproduction that prevents any serious reaction (usually) even in contact with the bloodstream. [Next page](https://boards.straightdope.com/t/why-dont-female-immune-systems-reject-sperm/622055.md?page=2)