Blood Type

A blood type is a label that tells what kind of blood a person has. Not all human blood is the same. Tiny markers sit on the surface of every red blood cell, and these markers come in different versions. Doctors use blood types to make sure that a person who needs new blood gets blood their body will accept.

The most common system is called ABO. It groups blood into four types: A, B, AB, and O. The letters stand for the kind of marker on the red blood cells. Type A has A markers. Type B has B markers. Type AB has both. Type O has neither.

There is also a second label: positive or negative. This is called the Rh factor. So a person might be A positive, O negative, AB positive, and so on. That gives eight common blood types in total. Around the world, O positive is the most common. AB negative is the rarest.

Why do blood types matter? Your immune system protects you by attacking anything that looks foreign. If a person with type A blood is given type B blood, their immune system sees the B markers as invaders. It attacks the new blood cells, and the person can get very sick. So before any blood transfusion, doctors check that the types match.

Type O negative blood is special. It has no A, B, or Rh markers, so almost anyone can receive it safely. People with O negative blood are called universal donors. Hospitals keep a careful supply of it for emergencies, when there is no time to test a patient first. People with type AB positive blood are universal receivers. They can accept blood from any other type.

You inherit your blood type from your parents, just like eye color or hair color. Each parent passes down one gene that helps decide the type. That is why kids in the same family can sometimes have different blood types.

Blood types were not understood until 1901, when an Austrian doctor named Karl Landsteiner figured out the ABO system. Before then, blood transfusions often killed the patient, and no one knew why. Landsteiner's discovery saved countless lives and won him the Nobel Prize. Scientists are still finding new blood group systems today. They have identified more than 40, though most people only ever need to know their ABO type and Rh factor.