# Why is rhesus incompatibility an issue, while there's no blood group incompatibility?

**URL:** <https://boards.straightdope.com/t/why-is-rhesus-incompatibility-an-issue-while-theres-no-blood-group-incompatibility/608148>\
**Category:** Factual Questions\
**Created:** [January 2, 2012, 3:37pm UTC](https://boards.straightdope.com/t/why-is-rhesus-incompatibility-an-issue-while-theres-no-blood-group-incompatibility/608148 "2012-01-02T15:37:42Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![clairobscur](https://avatars.discourse-cdn.com/v4/letter/c/839c29/32.png) [@clairobscur](https://boards.straightdope.com/u/clairobscur)\
**Post date:** [January 2, 2012, 3:37pm UTC](https://boards.straightdope.com/t/why-is-rhesus-incompatibility-an-issue-while-theres-no-blood-group-incompatibility/608148/1 "2012-01-02T15:37:42Z")

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I’m refering to parents having different rhesus factors. If I’m not mistaken, it’s only an issue if the mother is Rh- and the father Rh+ and normally only for the second and later pregnancies. Correct me if I’m wrong.

I believe I get how it works, but what I don’t get is why, say, an O+ mother having a B+ baby isn’t an issue at all.

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**Author:** ![Nunavut\_Boy](https://avatars.discourse-cdn.com/v4/letter/n/e95f7d/32.png) [@Nunavut\_Boy](https://boards.straightdope.com/u/Nunavut_Boy)\
**Post date:** [January 2, 2012, 5:13pm UTC](https://boards.straightdope.com/t/why-is-rhesus-incompatibility-an-issue-while-theres-no-blood-group-incompatibility/608148/2 "2012-01-02T17:13:21Z")

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The short answer is that it can be an issue. [ABO-HDN](http://en.wikipedia.org/wiki/Hemolytic_disease_of_the_newborn_(ABO)) wiki article should answer most of your questions.

The gist of it is that anti-A and anti-B are usually IgM antibodies, which do not cross the placenta, while anti-D (I guess you could call this anti-Rh factor) is IgG and crosses easily.