There are two main blood groupings in humans, called ABO and rhesus.

• Both are genetically determined.
• The ABO system has three gene variants known as alleles:
  • A, B and O.
  • Each of us inherits two, one from each parent.
  • A and B are dominant and O is recessive.
  • People who inherit two Os are blood group O.
  • Everybody else is either A (AA or AO), B (BB or BO) or AB.
• Rhesus is similar, but has only two alleles:
  • Rh+ (dominant) and Rh- (recessive).
  • The groupings are independent of each other so somebody who is A can be either rhesus positive or rhesus negative, for example.
• Blood types are expressed as molecules on the surface of red blood cells.
  • There are four types of these molecules:
    • O, A, B and Rh+ (Rh- is simply the absence of Rh+).
• Everybody has the O type regardless of their blood group, which is why O-negative blood can be transfused into anyone.
  • But the wrong blood type will provoke a violent immune response.
  • For example, somebody who is O or B being given type A blood.