In the past, there were many problems related to blood transfusions. Blood transfusions were even causing deaths. After the medical establishment recognized that all blood types were not compatible, and one may destroy another, an Austrian named Karl Landsteiner created this system of blood types:
Once the blood types were classified, it provided greater confidence to doctors who gave blood transfusions.
O negative blood type is highly sought after because it can be given to patients with other blood types without destruction.
Suellen Ocean is the author of Secret Genealogy – A How-to for Tracing Ancient Jewish Ancestry, Secret Genealogy II – Uncovering the Jewish Roots of Our Christian Ancestors and Secret Genealogy III – From Jewish Anglo-Saxon Tribes to New France Acadians. Available here:
Secret Genealogy II: http://www.amazon.com/Secret-Genealogy-II-Christian-Ancestors/dp/1484053222
Secret Genealogy III: http://www.amazon.com/Secret-Genealogy-III-Jewish-Anglo-Saxon-Acadians/dp/148407579X