An illustration showing six loops of protein in a red blood cell

Am I rhesus positive or rhesus negative?

Actually you are neither - rhesus is the wrong name for this particular blood group. For more surprising facts, read on

What does it actually mean to be Rh negative or positive? At its simplest, you are Rh positive if you possess the D antigen, a type of protein, on the external surface of your red cells. If you do not have this D antigen, you are Rh negative.

D is probably the single most important antigen that is expressed on the human red cell, after the ABO antigens. It is made up of 416 amino acids which pass through the red cell membrane 12 times (rather like a worm). Six loops of the protein can be detected on the external surface of the red cell, and both ends are on the inside of the red cell (see diagram above).

Rh in pregnancy

Whether or not you are Rh positive or Rh negative makes no difference to you in terms of your health and general well being, but it can make a difference if you require a transfusion or if you are an Rh negative woman carrying an Rh positive baby.

Normally people only make antibodies to the antigens they don't have, and only after they have been exposed to the "foreign" antigen (the ABO system is different in this respect). However, this happens only if you are a "responder" - and not everyone is a "responder".

"It is even rarer for pregnant Rh negative women to produce anti-D, even if they are carrying an Rh positive baby"

Suppose you are Rh negative (no D antigen on your red cells) and you are transfused with Rh positive blood. There is (approximately) an 80 per cent chance that you will "respond" and make anti-D (the antibody to D) because your immune system will recognise the Rh positive blood with the D antigen as "foreign".

If you have made anti-D, then the next time that you are transfused with Rh positive blood the anti-D will destroy this transfused blood.

In the case of an Rh negative woman carrying an Rh positive baby, because there is always a slight bleed from the baby's circulation into the mother's during pregnancy (this is perfectly normal), it can act like a transfusion of Rh positive blood into the mother.

An illustration showing 3 pregnant women.

She may produce anti-D in response. This antibody does not normally affect the pregnancy, but it can affect future pregnancies if the baby is Rh positive. The mother's anti-D can pass through the placenta and destroy the baby's red cells, causing serious harm to the baby. This has different names - blue baby syndrome and Rhesus baby syndrome - but the proper term is haemolytic disease of the foetus and newborn.

These days, it is very rare for Rh negative people to be transfused with Rh positive blood (and never, unless there is no alternative, for such blood to be transfused to a female of child-bearing age).

It is even rarer for pregnant Rh negative women to produce anti-D, because, in most cases, we can prevent this happening. We give the mother a dose of anti-D immunoglobulin and this clears any of the baby's Rh positive red cells from her circulation before her immune system has time to recognise these red cells as being "foreign". Thus, she does not produce anti-D of her own.

Not so simple

Unfortunately, humans being humans, Rh positive and Rh negative are not the only possibilities.

As the protein is 416 amino acids long, there is potential for one or more of these amino acids to be replaced by different amino acids, or, indeed for some of these amino acids to be missing completely. These are rare occurrences, but they do happen.

If an amino acid is replaced by another, and this amino acid happens to be in the part of the molecule that is not on the outside of the red cell membrane, we call this "Weak D". Because there is no change to the molecule on the outside of the red cell membrane, the immune system of people with Weak D does not recognise true Rh positive blood as foreign when it is transfused to such people, so will not be stimulated to make anti-D.

If, however, a particular amino acid is replaced by another, and this amino acid happens to be in one of the external loops, then the protein is different to the normal Rh positive protein. Such individuals have a "Partial D". If such individuals are transfused with normal Rh positive blood, then their immune system can recognise this protein as "foreign", and can sometimes produce an anti-D. The diagrams on pxx show how Partial D and Weak D work at the red cell level.

Let's put it this way

Malcolm Needs, Reference Service Manager, is an expert in blood groups. He has an unusual way of explaining the concept of weak and partial D.

"Imagine I have three cooker tops, and the tops represent the red cell membrane.

The first top (1) has many bottles of herbs and spices on it (representing the D antigen). This is the situation with the normal Rh Positive red cell.

The second cooker top (2) has fewer of these bottles. This is the situation with the Weak D red cell. There are fewer antigens, but they are identical in shape to the normal D antigens, and so this individual's immune system will not recognise the normal D antigens as 'foreign'.

Illustration of Weak and Partial D antigens

The third cooker top (3) has herb and spice bottles that are entirely a different shape. These are still D antigens, but are different from the normal D antigen. This is the situation with Partial D red cell. Because they are a different shape, if such an individual is transfused with normal Rh positive red cells, this individual's immune system can recognise the normal D antigen as 'foreign', and can produce an anti-D."

Malcolm adds that true Rh negatives would have no herb or spice bottles at all!

Weak D individuals are always treated as Rh positive, whereas Partial D individuals are treated as Rh positive when they are donating blood, but as Rh negative when receiving blood - which may explain why some of you appear to have a blood group that changes

What's in a name?

An old black and white photograph of Landsteiner and Wiener's laboratory

The name Rhesus or rhesus is actually a misnomer for the blood group called Rh.

The confusion began after two workers, Landsteiner and Wiener, described an antibody that reacted very like another antibody described by two other workers, Levine and Stetson about 70 years ago.

Landsteiner and Wiener injected the red cells of a rhesus monkey (Macaca rhesus) into rabbits and guinea pigs and found this created an antibody in their blood. Levine and Stetson found a similar antibody in humans. The presence of the antibody confirmed the presence of a new blood group, distinct from ABO, and this antibody was called anti-D.

But the two antibodies (and therefore blood groups) were found to be not identical. The one described by Levine and Stetson was renamed anti-LW, to recognise their work, whilst anti-D became the first antibody recognised in the Rh Blood Group System. In fact, we now know that anti-LW belongs to a completely different blood group system.

Do not worry if you get this wrong, as many very senior workers in the world of transfusion still refer to Rh as rhesus.