not all people are able to donate blood to all other people. testing for blood types usually prevents any bad reaction to a blood transfusion. however, there are special problems for anyone with rare blood groups. it is very important, in order to avoid any complications during a blood transfusion, to detect individuals with rare blood types. the usual tests for abo blood group system would show them as group o. it may not be possible to give a blood transfusion at all unless the repository has supply of blood which will not cause a reaction.it donates blood