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This question already has an answer here:

What are the popular theories regarding how our blood divided into 4 groups. Particularly I'm interested in whether this was originally linked to disease.

Thanks for your answers, useful articles and further reading would also appreciated.

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marked as duplicate by fileunderwater, Amory, Cornelius, WYSIWYG Sep 15 '15 at 18:45

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Hi and welcome. Your question is very interesting, however, the general expectation of SE is that you demonstrate that you have researched the topic of your question and have hit a wall in that search, or you are looking for expert advice to explain an advanced topic that you have found that you are having a difficult time explaining. Review How Do I Ask a Good Question for tips and general expectations for the site. Though I personally hope someone can provide an answer as I would be interested in this as well. $\endgroup$ – AMR Sep 15 '15 at 13:11
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    $\begingroup$ Possible duplicate of biology.stackexchange.com/q/31920/3340 $\endgroup$ – WYSIWYG Sep 15 '15 at 13:15
  • $\begingroup$ yep, sorry for duplicating and thanks for your answers! $\endgroup$ – dshulgin Sep 16 '15 at 8:58