My main purpose for knowing my blood type is in the event someone else NEEDS blood in an emergency, for surgery, etc…Other than curiosity than is little reason to know your blood type. Even in an emergency no hospital is going to go by what you tell them or what is written on a card. In an extreme emergency anyone can be given O negative blood while a type and crossmatch is done in the blood bank where results are checked and rechecked. In Viet Nam we had casualties with the wrong blood type marked on their dog tags. We never trusted dog tags or ID cards.