Skip to content
Know.itKnow.it
Create an animated lesson
On request

This article is free to read. Creating a lesson uses your daily or stored credits; review the language and settings before starting.

How do blood types work?

Medicine & HealthLesson on requestFree to readयह पेज हिंदी में पढ़ें

Quick answer

Blood types are labels for the antigens on your red blood cells. The ABO system uses A and B antigens, and the Rh system uses the D antigen. Your plasma naturally carries antibodies against the A or B antigens you lack, so a mismatched transfusion can cause the immune system to destroy the donated cells.

Blood types are determined by antigens, which are marker molecules on the surface of red blood cells. The ABO and Rh systems are the two most important, and mismatches between donor and recipient can trigger dangerous immune reactions.

Step by step

  1. Genes inherited from both parents determine which antigens your red blood cells can build.
  2. The ABO gene adds either A, B, both, or neither antigen to the cell surface.
  3. The Rh gene adds the D antigen, making the type positive, or leaves it off, making it negative.
  4. Your immune system learns to tolerate your own antigens and makes antibodies against foreign ones.
  5. During a transfusion, the recipient's antibodies check the donor cells for unfamiliar antigens.
  6. If the antigens do not match, antibodies bind the donor cells and trigger their destruction.

Key things to understand

  1. Red blood cells carry antigens, which are sugar or protein markers on their outer membrane.
  2. The ABO system has two possible antigens, A and B, giving four main types: A, B, AB, and O.
  3. The Rh system adds a second marker, the D antigen, making each ABO type positive or negative.
  4. Plasma contains antibodies that attack antigens the body does not recognise as its own.
  5. A transfusion must match antigens so the recipient's antibodies do not destroy the donated cells.

Labelled diagram

  1. Red blood cell

    A flexible disc that carries oxygen and displays marker molecules on its surface.

  2. ABO antigens

    Sugar chains called A and B; you may have A, B, both, or neither.

  3. Rh antigen

    The D protein marker; present makes you positive, absent makes you negative.

  4. Plasma antibodies

    Proteins that recognise and attack antigens your own cells do not carry.

  5. Transfusion match

    Donor cells must lack antigens that the recipient's antibodies target.

How ABO and Rh antigens determine blood type

Real-world examples

  • A patient with type A blood can receive A or O red cells but not B or AB cells.
  • In an emergency, hospitals may give O negative red cells before the patient's type is known.
  • An Rh-negative mother may receive an injection to stop her immune system making anti-D antibodies.
  • Organ transplant teams also match tissue antigens, a principle that began with blood typing.

Common mistake

✗ Myth: People with type O blood can receive any blood type because O is universal.

✓ Fact: O negative is the universal donor, not recipient: type O plasma carries both anti-A and anti-B, so it accepts only O red cells. AB positive people are the universal recipients, as their plasma lacks anti-A and anti-B.

People also ask

What determines your blood type?
Your blood type is determined by genes you inherit from your parents. These genes control which antigens, such as A, B, and the D antigen, appear on your red blood cells. The combination of antigens you carry defines your ABO and Rh type.
Why is O negative blood so important?
O negative red cells carry no A, B, or D antigens, so they can be given to almost anyone without triggering an immune attack. This makes them vital in emergencies when a patient's blood type is unknown and there is no time to test.
What happens if you get the wrong blood type?
If the donor blood carries antigens the recipient's antibodies recognise, the antibodies bind to the donor cells. This causes the cells to clump and break apart, releasing substances that can lead to fever, kidney damage, and in severe cases a life-threatening reaction.
Which blood type is the universal donor?
O negative red blood cells are called the universal donor type because they carry neither A, B, nor RhD antigens. That means a recipient's antibodies have nothing to attack, so the cells can be given to almost anyone in an emergency.
Can a mother and baby have incompatible blood types?
Yes. If an Rh-negative mother carries an Rh-positive baby, some of the baby's cells can enter her blood and trigger antibodies. These antibodies may then attack the baby's red cells in a later pregnancy, a condition called haemolytic disease of the newborn.
How common is each blood type?
Frequencies vary by region. In many populations O positive is the most common, followed by A positive and B positive, while AB negative is one of the rarest. Blood banks track local frequencies to manage supply.
Can blood type change over a person's life?
No, a person's ABO and Rh type is set by their genes and stays the same. The only exception is after certain medical procedures such as a bone marrow transplant, where the donor's stem cells can produce a different blood type.

Part of the guide: How Your Body Works →