Yrys said:
Civy question : Is it thuth ? And if yes, why ?
Simple answer: Yes, because the ID card/tag may be wrong.
The more detailed but still simple answer: Blood typing is a laboratory test done to determine a person's blood type. If the person needs a blood transfusion, another test called crossmatching is done after the blood is typed to find blood from a donor that the person's body will accept.
Blood typing and crossmatching are most commonly done to make certain that a person who needs a transfusion will receive blood that matches (is compatible with) his own. People must receive blood of the same blood type, otherwise, a serious, even fatal, transfusion reaction can occur.
Crossmatching is the final step in pretransfusion testing. It is commonly referred to as compatibility testing, or "Type and Cross." Before blood from a donor and the recipient are crossmatched, both are ABO and Rh typed. In addition, antibody screening is done to look for antibodies to certain Rh, Duffy, MNS, Kell, Kidd, and P system antigens. If an antibody to one of these antigens is found, only blood without that antigen will be compatible in a crossmatch. This sequence must be repeated before each transfusion a person receives.
To begin the crossmatch, blood from a donor with the same ABO and Rh type as the rcipient is selected. If an unexpected antibody is found in either the patient or the donor, the blood bank does further testing to make sure the blood is compatible.
In an emergency, when there is not enough time for blood typing and crossmatching, O red blood cells may be given, preferably Rh-negative. O blood type is called the universal donor because it has no ABO antigens for a patient's antibodies to attack. In contrast, AB blood type is called the universal recipient because it has no ABO antibodies to attack the antigens on transfused red blood cells. If there is time for blood typing, red blood cells of the recipient type (type specific cells) are given. In either case, the crossmatch is continued, even though the transfusion has begun.
If you are interested in some aspects of blood trasnsfusion in a military setting: (it's not just about sticking a needle in a guy's arm and letting red stuff flow in)
Fresh Whole Blood Transfusion: A Controversial Military Practice http://www.usaisr.amedd.army.mil/gwot/FWB%20-%20Kauvar%202006%20J%20Trauma.pdf