While everyones blood is made up of the same basic parts, theres actually a lot of variety in the kinds of blood that exist. There are eight different blood types, and the type you have is determined by genes you inherit from your parents.
The Basic Elements of Blood
Most people have about 4-6 liters of blood. Your blood is made up of different kinds of cells that float in a fluid called plasma:
Your red blood cells deliver oxygen to the various tissues in your body and remove carbon dioxide.
Your white blood cells destroy invaders and fight infection.
Your platelets help your blood to clot.
Your plasma is a fluid made up of proteins and salts.
What makes your blood different from someone elses is your unique combination of protein molecules, called antigens and antibodies.
Antigens live on the surface of your red blood cells. Antibodies are in your plasma.
The combination of antigens and antibodies in your blood is the basis of your blood type.
The Different Blood Types
There are four major blood groups and eight different blood types. Doctors call this the ABO Blood Group System.
The groups are based on whether or not you have two specific antigens -- A and B:
Group A has the A antigen and B antibody.
Group B has the B antigen and the A antibody.
Group AB has A and B antigens but neither A nor B antibodies.
Group O doesnt have A or B antigens but has both A and B antibodies.
Theres also a third kind of antigen called the Rh factor. You either have this antigen (meaning your blood type is Rh+ or positive), or you dont (meaning your blood type is Rh- or negative). So, from the four blood groups, there are eight blood types:
A positive or A negative
B positive or B negative
AB positive or AB negative
O positive or O negative
Why Does Blood Type Matter?
Blood groups were discovered in 1901 by an Austrian scientist named Karl Landsteiner. Before that, doctors thought all blood was the same, so many people were dying from blood transfusions.