As I understand it, blood types A and B denote the presence of the A and B factor respectively; type AB denotes the presence of each; and type O the absence of each.
A transfusion-recipient will have adverse reaction to any received blood that contain a factor not in his/her own blood, a "strange" factor if you wille Thus types A and AB cannot be given to a person of type B or O; and types B and AB cannot be given to one of type A or O; and blood-type O is the "universal donor".
Rh+ and Rh- denote the presence or absence of the Rh factor, particularly important in pregnancy. When an Rh- woman (such as my wife) delivers an Rh+ baby, any cross-flow of Rh+ blood from baby to mother may cause her body to develop antibodies to Rh+, and in any furture pregnancy those antibodies could severely attack a future Rh+ fetus she may carry. Thus, it is routine for an Rh+ woman, immediately after delivery, to be given treatment to suppress the development of those antibodies. Routine -- but my wife's experience is that she has had to remind the hospital personnel.
