What is a Blood Type?

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.
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