You want to know how people know Jesus’ blood type was AB+.

The easy answer is—they don’t. We aren’t sure at all His blood type was AB+. But there are bits of information that lead us in this direction.

First, no matter how vehemently folks demean the legitimacy of the famous Shroud of Turin, we don’t have a definitive answer on it. Even though authors on, e.g., Rational/Wiki, try to shame people interested in the shroud by calling them “Shroudies” and hypocrites and nitpickers, the shroud continues to be investigated by scientists, both Christian and non-Christian, and the issue is still not solved. You can read the news and try to decide for yourself at The Shroud of Turin Website (a website moderated by an Orthodox Jew!). Suffice it to say, the shroud has been tested and the result (so far) indicates a blood type of AB, although it can’t be ascertained (yet) if that’s human blood or not, for technical reasons.

Second, there have been numerous eucharistic miracles recorded, some in fairly recent history. These entail consecrated hosts displaying blood and tissue (body and blood) in place of the unleavened bread. Unlike apparition-type miracles (e.g., Lourdes, Fatima), eucharistic miracles are fairly easy to analyze, since you have blood and tissue present right there for the lab to test. In recent cases where the Church has allowed testing, the blood type is AB+.

So whether you believe in God or Jesus or miracles, the fact is that the best we can determine right now is that Jesus’ blood type is AB or AB+, if indeed these miracles and the shroud contain His blood.

Of course, always remember that “for those who believe, no proof is necessary. For those who don’t believe, no proof is possible.” —Stuart Chase

But remember, “there are things you know about, and things you don’t, the known and the unknown, and in between are the doors.” —Ray Manzarek, co-founder of the Doors.

I guess at this point, it depends on whether or not you want to keep knocking on those doors or just have an opinion and stick to it no matter what.

Hope that’s helpful.

[ADDENDUM: As I pondered this question over time a theological thought occurred to me: Type AB+ is considered the “universal recipient,” which seems to be consistent with Jesus’ role of receiving all of our sins—universally receiving our sins upon the cross. Just an interesting correlation.]

Also from another poster:

If I’m AB+, is there still any point in donating blood? YES, tons, actually.

When people talk about O- being the “universal donor” they mean universal for red blood cells, not for all types of blood products derived from donations.

You, dear AB+ friend, are actually the universal donor for both platelets and plasma!

Additionally, whenever possible, blood products are type-matched rather than using up the “universal.” Your red blood cells can still go to approximately 3% of the population, and your platelets and plasma to nearly 100% of the population.

https://www.quora.com/What-are-some-amazing-facts-about-AB-positive-blood-group