The Rhesus or Rh factor is present in all red blood cells and was detected in 1940.

It was named from the Rhesus monkey on which the experiments were conducted.

There are three pairs of Rh factors, or antigens, and these are determined by six genes — Ñ, c; D, d; E, e — carried on a pair of chromosomes. One chromosome comes from the mother and one from father.

However, it is the D antigen which alone determines whether a person is Rh positive or Rh negative. Eighty-five per cent of us are Rh positive, that is we have the D gene.

When an Rh negative woman is pregnant with an Rh positive foetus, the red cells containing this Rh factor can leak into the mother’s circulation. And stimulate the production of antibodies.

This leakage may occur at any time during pregnancy but is more likely at the time of delivery or during a spontaneous miscarriage or induced abortion.

This sensitisation is more likely if the mother’s blood and that of the baby are ABO compatible. The ABO is the main blood grouping of which there are four — À, Â, AB and O.

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