Knowing their blood type can help people better understand their health and be more informed about potential treatments. A person can find out their blood type with a blood test or other methods.

The presence or absence of substances called antigens helps determine blood type. Antigens are involved in triggering the body’s immune response.

A person may have A antigens, B antigens, both, or neither on the surface of their red blood cells. They may also have a protein called the Rhesus (Rh) factor.

In this article, we explain how a person can identify their blood type. We also investigate what a blood type is, how it comes to be, and how it can affect blood transfusions.

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People who know their blood type might also have a better understanding of their health.

A person usually visits a clinic or doctor’s office to find out their blood type. There, a doctor or nurse draws a blood sample and sends it to a lab for testing.

However, there are ways of determining blood type at home.

By drawing blood

A person can identify their blood type at home using a rapid blood typing kit.

Using the kit requires a person to prick a finger with a needle. The kit comes with a card that contains chemicals known as reagents. These test for the presence of antibodies and the Rh factor.

When one or more of the blood type antibodies or the Rh factor is present in the blood sample, the test produces a “clumping” of the blood. Typically, if clumping does not occur, the person’s blood type is O-negative.

Without drawing blood

A person may be able to use a saliva sample to test for their blood type. Around 80% of people produce the relevant antigens in their saliva.

According to 2018 research, if a person secretes these antigens in their saliva, a dried saliva sample can reliably indicate their blood type.

However, it is important to note that not everyone secretes these antigens, and this method does not necessarily indicate the presence or absence of the Rh factor.

A person may find out their blood type by donating blood.

Nonprofit organizations such as the American Red Cross collect blood from volunteers for use at hospitals and in emergency situations around the world.

By participating in a blood drive, for example, a person will donate 1 pint of blood, which can take 4–6 weeks to replenish completely.

A person may then receive a donor card, which may indicate their blood type.

A person does not need to know their blood type to donate blood.

The first stage in identifying blood type involves a test called ABO typing. This will determine which of the following four main blood types a person has:

  • A: the presence of just the A antigen
  • B: the presence of just the B antigen
  • AB: the presence of both antigens
  • O: the presence of neither antigen

The second stage involves identifying the presence (+) or absence (-) of the Rh factor in the person’s blood.

Incorporating the four ABO types and the two Rh types, there are eight common blood types:

  • A+
  • A-
  • B+
  • B-
  • AB+
  • AB-
  • O+
  • O-

A person inherits their blood type from their biological parents.

A parent with type A blood can either pass on the A antigen or no antigen at all. If the other parent has type B blood, they can pass on the B antigen or no antigen. Depending on the combination, their child could have A, B, AB, or O blood.

If both parents have O blood, the child can only have O blood.

A person inherits the Rh factor in the same way.

A blood transfusion typically occurs when a person has experienced trauma or surgery that has resulted in extreme blood loss.

When a person receives a blood transfusion, it is essential to match the blood type of the donor to that of the recipient. The reason is that the body develops antibodies against any ABO antigens that are not naturally present in the blood.

If a person receives blood from a donor with a different blood type, it will contain these antigens, and the recipient’s body will reject the donated blood.

A doctor may describe this rejection as an acute immune hemolytic reaction or an acute hemolytic transfusion reaction.

This reaction can be dangerous and even fatal. As the recipient’s antibodies attack the blood cells from the transfusion, the cells break open, releasing substances into the bloodstream that can cause harm.

The kidneys can become damaged as a result, and healthcare professionals will need to stop the transfusion immediately.

Some symptoms of an acute hemolytic transfusion reaction include:

If a person’s blood is O-negative, they are a universal donor. O-negative blood does not typically cause adverse reactions during transfusions because it contains neither antigen and no Rh factor.

As the American Cancer Society notes, if a person has:

  • Type A blood: They are unable to receive type B or AB red blood cells.
  • Type B blood: They are unable to receive type A or AB red blood cells.
  • Type AB blood: They can receive type A, B, AB, or O red blood cells.

While it is not often necessary for a person to know their blood type, this information can be useful.

A person’s blood type is inherited, and there are a few ways to identify it, including visiting a doctor’s office or clinic, donating blood, and using a home testing kit.