# Why don't human genetic chimeras have problems with their immune system? **URL:** \ **Category:** Factual Questions\ **Created:** [October 24, 2014, 8:20am UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335 "2014-10-24T08:20:05Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![grude](https://avatars.discourse-cdn.com/v4/letter/g/e47774/32.png) [@grude](https://boards.straightdope.com/u/grude)\ **Post date:** [October 24, 2014, 8:20am UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/1 "2014-10-24T08:20:05Z")
The wikipedia article only addresses it with a short sentence saying it isn’t a problem usually. There are a lot of denser sources but most go way over my head. I did think of the idea that incompatible chimeras immune wise simply don’t survive the fetus stage, but I’m not even sure at what point a fetus’s immune system starts functioning. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [October 24, 2014, 9:29am UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/2 "2014-10-24T09:29:22Z")
For the same reason getting organs from a close relative often works fine. ---
**Author:** ![grude](https://avatars.discourse-cdn.com/v4/letter/g/e47774/32.png) [@grude](https://boards.straightdope.com/u/grude)\ **Post date:** [October 24, 2014, 9:35am UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/3 "2014-10-24T09:35:34Z")
> [@naita](#): > > For the same reason getting organs from a close relative often works fine. Sure but I thought often even siblings can be not compatible immune wise. I mean if even a sibling from the two same bio parents can be incompatible for organ donation, I wonder why a chimera could be fine. ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [October 24, 2014, 10:14am UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/4 "2014-10-24T10:14:40Z")
> [@grude](#): > > I did think of the idea that incompatible chimeras immune wise simply don’t survive the fetus stage, Probably this, but you’ve piqued my interest and I’d love to know the actual answer. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [October 24, 2014, 11:41am UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/5 "2014-10-24T11:41:11Z")
Natural chimeras (organ transplant recipients being artificial ones) have multiple genomes from the get go, so as the immune system develops it learns that both genomes are “self”. In theory, if you did an organ transplant early enough in pregnancy the recipient’s immune system would likewise come to recognize that foreign DNA as “self”. ---
**Author:** ![Fiveyearlurker](https://avatars.discourse-cdn.com/v4/letter/f/da6949/32.png) [@Fiveyearlurker](https://boards.straightdope.com/u/Fiveyearlurker)\ **Post date:** [October 24, 2014, 1:03pm UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/6 "2014-10-24T13:03:14Z")
Central tolerance. During immune cell maturation, T cells and B cells are exposed to self antigens, and any that respond are deleted. At least in the cartoon text book version of events. Reality is a bit messier. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [October 24, 2014, 8:24pm UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/7 "2014-10-24T20:24:10Z")
Is it also possible that there is sample selection? I.e., those chimeras that have auto-rejection don’t survive to be born. ---
**Author:** ![Chimera](https://avatars.discourse-cdn.com/v4/letter/c/8e8cbc/32.png) [@Chimera](https://boards.straightdope.com/u/Chimera)\ **Post date:** [October 24, 2014, 8:58pm UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/8 "2014-10-24T20:58:15Z")
ummm… nevermind. 😉 ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [October 25, 2014, 8:53am UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/9 "2014-10-25T08:53:18Z")
> [@Chimera](#): > > ummm… nevermind. 😉 Awww, I’m disappointed. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [October 25, 2014, 3:51pm UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/10 "2014-10-25T15:51:38Z")
> [@naita](#): > > Awww, I’m disappointed. Obviously, **Chimera** had one part that wanted to post more but another part overruled that. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [October 25, 2014, 4:12pm UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/11 "2014-10-25T16:12:06Z")
> [@Fiveyearlurker](#): > > Central tolerance. During immune cell maturation, T cells and B cells are exposed to self antigens, and any that respond are deleted. > > At least in the cartoon text book version of events. Reality is a bit messier. Isn’t central tolerance mediated by contact with specific cell types in the marrow and thymus? Depending how a chimera forms, some maturing T and B cells might only encounter other cells from the same genetic type. If that happened there would not be any negative selection against “self”-reactivity to antigens from the other genetic type. (Caveat: my knowledge of immunology is very, very spotty.) ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [October 25, 2014, 5:49pm UTC](https://boards.straightdope.com/t/why-dont-human-genetic-chimeras-have-problems-with-their-immune-system/702335/12 "2014-10-25T17:49:38Z")
> [@ftg](#): > > Is it also possible that there is sample selection? I.e., those chimeras that have auto-rejection don’t survive to be born. Yes. If an organism has this much of a fundamental immune conflict during natal development or shortly after birth, it simply wouldn’t survive into childhood. There are some diseases hypothesized to result from or exacerbated by chimerism, and many known to be directly caused by mosaicism (an organism that contains cells of different genotypes derived from one zygote). Stranger