The exact composition of blood varies between people. This difference in structure is what makes a person’s blood type. Most people use the ABO system to group blood types, and rhesus factor, which is either positive or negative.

An individual’s blood type depends on which genes they inherited from their parents.

ABO is the best-known system for grouping blood types, though there are other methods. There are four major categories within the ABO group: A, B, O, and AB. Within these groups, there are a further eight blood types.

Every 2 seconds, a person in the United States needs blood. When a person needs a transfusion, doctors must give them the right type. The wrong blood type can trigger an adverse reaction that could be life-threatening.

How does blood work, and what problems can arise?

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The main components of blood are:

  • red blood cells, which carry oxygen around the body
  • white blood cells, which play a crucial role in the immune system
  • plasma, which is a yellowish liquid that contains proteins and salts
  • platelets, which enable clotting

The blood group will depend on which antigens are on the surface of the red blood cells.

Antigens are molecules. They can be either proteins or sugars. The types and features of antigens can vary between individuals, due to small genetic differences.

The antigens in blood have various functions, including:

  • transporting other molecules into and out of the cell
  • maintaining the structure of red blood cells
  • detecting unwanted cells that could cause illness

Scientists use two types of antigens to classify blood types:

  • ABO antigens
  • Rh antigens

Antigens and antibodies play a role in the immune system’s defense mechanism.

White blood cells produce antibodies. These antibodies will target an antigen if they consider it a foreign object.

This is why it is essential to match blood types when a person needs a transfusion.

According to the American Red Cross, If a person receives red blood cells with antigens that are not already present in their system, their body will reject and attack the new red blood cells.

This can cause a severe and possibly life-threatening reaction.

How much blood is in the human body?

The ABO blood group system classifies blood types according to the different types of antigens in the red blood cells and antibodies in the plasma.

They use the ABO system alongside the RhD antigen status to determine which blood type or types will match for a safe red blood cell transfusion.

There are four ABO groups:

Group A: The surface of the red blood cells contains A antigen, and the plasma has anti-B antibody. Anti-B antibody would attack blood cells that contain B antigen.

Group B: The surface of the red blood cells contains B antigen, and the plasma has anti-A antibody. Anti-A antibody would attack blood cells that contain A antigen.

Group AB: The red blood cells have both A and B antigens, but the plasma does not contain anti-A or anti-B antibodies. Individuals with type AB can receive any ABO blood type.

Group O: The plasma contains both anti-A and anti-B antibodies, but the surface of the red blood cells does not contain any A or B antigens. Since these antigens are not present, a person with any ABO blood type can receive this type of blood.

Rhesus factor

Some red blood cells have Rh factor, also known as the RhD antigen. Rhesus grouping adds another dimension.

If the red blood cells contain the RhD antigen, they are RhD positive. If they do not, they are RhD negative.

Understanding ABO and Rhesus

Doctors need to take into account both ABO and Rh when considering blood types. This means there are eight main blood types in the ABO/Rh blood group system. Some are more common than others.

According to the American Association of Blood Banks, the distribution of blood types in the U.S. are as follows:

ABO blood typePercentage of people
A-positive (A+)30%
A-negative (A-)6%
B-positive (B+)9%
B-negative (B-)2%
AB-positive (AB+)4%
AB-negative (AB-)1%
O-positive (O+)39%
O-negative (O-)7%

Around 82% of people in the U.S. have Rh-positive blood. The rarest blood group type is AB negative.

These are the main types. Within the eight main groups, are there also many lesser-known and less common blood types.

Universal donor and universal recipient

O negative blood contains no A, B, or RhD antigens. Almost anyone with any blood type can receive these red blood cells. A person with group O negative blood is a universal donor.

  • A person with O-negative blood can donate to almost anyone.
  • A person with Rh-negative blood can donate to a person with Rh-negative or Rh-positive blood.
  • A person with Rh-positive blood can only donate to someone with Rh-positive blood.

As a result, there is a high demand for O negative blood, even though fewer than 10% of the U.S. population has this type.

The rules for plasma are the opposite of those for Rh. A universal plasma donor will have type AB blood.

Before a person receives donated blood, doctors will check that this blood is compatible. Giving someone the wrong blood type can lead to potentially life-threatening reactions and complications.

If someone with group B antigen receives red blood cells from someone with group A antigen, their body will launch an immune response and reject the transfusion. The anti-A antibody in the recipient’s plasma will attack and destroy the A antigen donor red blood cells.

When the recipient’s plasma attacks and destroys the donor cells, the blood may clump, or agglutinate. This can lead to blood clots, which can obstruct blood vessels. If they break, hemoglobin can leak out, and this can be toxic.

Other possible adverse effects include allergic reactions and anaphylaxis. In some cases, the body can cope, but others can be life-threatening.

Some reactions occur at once, while others can take up to 28 days to appear.

In addition, blood can sometimes contain unexpected antibodies, viruses, or parasites. The donor may not have symptoms, but they can affect a recipient’s health.

Doctors and other specialists conduct stringent testing and screening before a person can receive donated blood, plasma, or other blood products.

Learn more about the benefits and risks of donating blood and the side effects and risk of donating plasma.

If two parents have different blood types, the mother will not necessarily have the same blood type or Rh factor as the child.

If the mother has Rh-negative blood, and the child has Rh-positive, this can pose a risk during pregnancy and delivery.

A small number of red blood cells from the fetus’ circulation can cross the placenta and enter the mother’s bloodstream. Anti-RhD antibody can then develop in the mother’s plasma, in a process known as sensitization.

A problem can arise if this antibody then detects a “foreign” antigen in the fetus’ blood cells. The antibodies may start to attack the fetus’ red blood cells as a defense mechanism.

In some cases, severe jaundice can result, and possibly brain damage.

An injection of anti-RhD immune globulin G can help prevent the mother from producing this antibody and reduce the impact of a sensitizing event on the fetus.

If a woman has Rh-negative blood, a doctor may administer anti-D immunoglobulin at 28 weeks and 34 weeks as a preventive measure, according to the World Health Organization (WHO).

Blood testing during pregnancy can predict possible risks by checking whether the fetus’ blood type is compatible with the mother’s.

Blood donation resources

To discover more evidence-based information and resources for donating blood, visit our dedicated hub.

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A blood test can determine an individual’s blood type.

To test blood, a healthcare provider will take a small sample, usually from the person’s arm.

In a lab, a technician mixes the individual’s blood with three different substances to see how they react. Each substance will contain A antibodies, B antibodies, or Rh factor.

The antibodies will cause a different reaction in each case. If the blood is incompatible, it will clump. Observing these reactions will enable the technician to identify a person’s blood type.

Before a person can receive donated blood, the technician will test the reaction by mixing a sample of the donor’s blood with that of the recipient.

Specialist technicians carefully test all blood and blood products before use.

The ABO system is the best known way of classifying blood types. In this system, there are eight main types. O positive is the most common, and AB negative is the rarest.

If a person needs a blood transfusion, their blood type must be compatible with that of the donor to avoid complications.

Blood donations save lives every day, but receiving the wrong type of blood can lead to life-threatening consequences.

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