What blood type is Rh negative

Antibodies and antigens

Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma. Your blood group is identified by antibodies and antigens in the blood.

Antibodies are proteins found in plasma. They're part of your body's natural defences. They recognise foreign substances, such as germs, and alert your immune system, which destroys them.

Antigens are protein molecules found on the surface of red blood cells.

The ABO system

There are 4 main blood groups defined by the ABO system:

• blood group A – has A antigens on the red blood cells with anti-B antibodies in the plasma
• blood group B – has B antigens with anti-A antibodies in the plasma
• blood group O – has no antigens, but both anti-A and anti-B antibodies in the plasma
• blood group AB – has both A and B antigens, but no antibodies

Blood group O is the most common blood group. Almost half of the UK population (48%) has blood group O.

Receiving blood from the wrong ABO group can be life-threatening. For example, if someone with group B blood is given group A blood, their anti-A antibodies will attack the group A cells.

This is why group A blood must never be given to someone who has group B blood and vice versa.

As group O red blood cells do not have any A or B antigens, it can safely be given to any other group.

The NHS Blood and Transplant (NHSBT) website has more information about the different blood groups.

The Rh system

Red blood cells sometimes have another antigen, a protein known as the RhD antigen. If this is present, your blood group is RhD positive. If it's absent, your blood group is RhD negative.

This means you can be 1 of 8 blood groups:

• A RhD positive (A+)
• A RhD negative (A-)
• B RhD positive (B+)
• B RhD negative (B-)
• O RhD positive (O+)
• O RhD negative (O-)
• AB RhD positive (AB+)
• AB RhD negative (AB-)

About 85% of the UK population is RhD positive (36% of the population has O+, the most common type).

In most cases, O RhD negative blood (O-) can safely be given to anyone. It's often used in medical emergencies when the blood type is not immediately known.

It's safe for most recipients because it does not have any A, B or RhD antigens on the surface of the cells, and is compatible with every other ABO and RhD blood group.

The NHS Blood and Transplant (NHSBT) website has more information about the Rh system.

Blood group test

To work out your blood group, your red cells are mixed with different antibody solutions. If, for example, the solution contains anti-B antibodies and you have B antigens on your cells (you're blood group B), it will clump together.

If the blood does not react to any of the anti-A or anti-B antibodies, it's blood group O. A series of tests with different types of antibody can be used to identify your blood group.

If you have a blood transfusion – where blood is taken from one person and given to another – your blood will be tested against a sample of donor cells that contain ABO and RhD antigens. If there's no reaction, donor blood with the same ABO and RhD type can be used.

Pregnancy

Pregnant women are always given a blood group test. This is because if the mother is RhD negative but the child has inherited RhD-positive blood from the father, it could cause complications if left untreated.

RhD-negative women of child-bearing age should always only receive RhD-negative blood.

Read more about Rhesus disease.

Giving blood

Most people are able to give blood, but only 1 in 25 people actually do. You can donate blood if you:

Rhesus disease is caused by a specific mix of blood types between a pregnant mother and her unborn baby.

It can only occur where all of the following happen, where the:

• mother has a rhesus negative (RhD negative) blood type
• baby has a rhesus positive (RhD positive) blood type
• mother has previously been exposed to RhD positive blood and has developed an immune response to it (known as sensitisation)

Blood types

There are several different types of human blood, known as blood groups, with the 4 main ones being A, B, AB and O. Each of these blood groups can either be RhD positive or negative.

Whether someone is RhD positive or RhD negative is determined by the presence of the rhesus D (RhD) antigen. This is a molecule found on the surface of red blood cells.

People who have the RhD antigen are RhD positive, and those without it are RhD negative. In the UK, around 85% of the population are RhD positive.

How blood types are inherited

Your blood type depends on the genes you inherit from your parents. Whether you're RhD positive or negative depends on how many copies of the RhD antigen you've inherited. You can inherit one copy of the RhD antigen from your mother or father, a copy from both of them, or none at all.

You'll only have RhD negative blood if you don't inherit any copies of the RhD antigen from your parents.

A woman with RhD negative blood can have an RhD positive baby if her partner's blood type is RhD positive. If the father has two copies of the RhD antigen, every baby will have RhD positive blood. If the father only has one copy of the RhD antigen, there's a 50% chance of the baby being RhD positive.

Sensitisation

An RhD positive baby will only have rhesus disease if their RhD negative mother has been sensitised to RhD positive blood. Sensitisation occurs when the mother is exposed to RhD positive blood for the first time and develops an immune response to it.

During the immune response, the woman's body recognises that the RhD positive blood cells are foreign and creates antibodies to destroy them.

In most cases, these antibodies aren't produced quickly enough to harm a baby during the mother's first pregnancy. Instead, any RhD positive babies the mother has in the future are most at risk.

How does sensitisation occur?

During pregnancy, sensitisation can happen if:

• small numbers of foetal blood cells cross into the mother's blood
• the mother is exposed to her baby's blood during delivery
• there's been bleeding during the pregnancy
• an invasive procedure has been necessary during pregnancy – such as amniocentesis, or chorionic villus sampling (CVS)
• the mother injures her abdomen (tummy)

Sensitisation can also occur after a previous miscarriage or ectopic pregnancy, or if a RhD negative woman has received a transfusion of RhD positive blood by mistake (although this is extremely rare).

How sensitisation leads to rhesus disease

If sensitisation occurs, the next time the woman is exposed to RhD positive blood her body will produce antibodies immediately.

If she's pregnant with an RhD positive baby, the antibodies can lead to rhesus disease when they cross the placenta and start attacking the baby's red blood cells.

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