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【背景】
血型分类:
一般常见A、B、AB和O四种，另外还有RH、MNS、P等极为稀少的10余种血型系统。其中，AB型可以接受任何血型的血液输入，因此被称作万能受血者，O型可以输出给任何血型的人体内，因此被称作万能输血者、异能血者、实际上，不同血型之间的输送，一般只能小量的输送，不能大量。要大量输血的话，最好还是相同血型之间为好。
狭义地讲，血型专指红细胞抗原在个体间的差异；但现已知道除红细胞外，在白细胞、血小板乃至某些血浆蛋白，个体之间也存在着抗原差异。因此，广义的血型应包括血液各成分的抗原在个体间出现的差异。通常人们对血型的了解往往仅局限于ABO血型以及输血问题等方面，实际上，血型在人类学、遗传学、法医学、临床医学等学科都有广泛的实用价值，因此具有着重要的理论和实践意义，同时，动物血型的发现也为血型研究提供了新的问题和研究方向。
【原文】
Yael: I just read about an exciting technique. Researchers found a way to convert types A and B blood into type O blood--the type that can be transfused into any patient. This advance could help avoid blood shortages in the future.
Don: Wow, that is exciting. But what makes type O blood so special?
Y: Blood types result from different carbohydrates, called antigens(抗原), on the surface of human blood cells. Your blood "type" denotes the kinds of antigens you have on the surface of your cells. People with type A blood have A antigens; those with type B have B antigens. If you have both kinds of antigens, you're type AB; if you have neither one, you're type O.
Your immune system recognizes the antigens on your blood as "self", but attack cells with "foreign" antigens. So if you ever need a blood transfusion, it's important that you only receive blood compatible with your own type.
D: I see. So if a type A person receives type B or AB blood, her immune system will see the B-antigens as foreign, and attack the donor blood cells?
Y: Exactly. But type O is called the "universal donor" since it doesn't have any antigens that would set off the body's defense system.
A team of Danish scientists has identified enzymes that remove the A and B antigens from blood cells. The enzymes, which were extracted from a bacteria and a fungus, essentially change any donated blood into type O.
Clinical trials are underway to test whether the treated blood is safe and effective. If so, the technology could help save lives, since type O blood is such a precious commodity.
