Blood Types

There are 8 main types of blood separated into 4 groups. The groups are A, B, AB, and O. They are grouped together by the presence or absence of an antigen. Antigens are substances within the blood that cause our immune systems to create antibodies. These antibodies kill anything the immune system thinks is a threat.

The specific antigens that create the different blood types are found on the surface of red blood cells and are known as type A and type B. They are separated by the presence of another type of antigen known as rH factor. If this rH antigen is present, blood is considered positive, if absent, negative.

Someone that has type A antigens and rH factor is considered type A+. If someone has both types of antigens and no rH factor would be type AB- blood. If no A or B antigens then it is type O.

All of this matters because of those antibodies your immune system creates. Someone with type A blood will have antibodies for type B, and someone with type B will have antibodies for type A. Type O has antibodies for both A and B. If you were to give type B blood to someone who was type A, their antibodies would attack the type A red blood cells causing very unwanted side effects, including possible death.

The two main types of cells within the blood are red and white. Red blood cells make up nearly 45% of your blood volume. White blood cells make up less than 1%. What is left over is blood plasma at approximately 55% of blood volume.

Red blood cells and most white blood cells are predominantly created within the bone marrow of large bones. White blood cell production is controlled within the immune system.

Blood Type Defined

Blood consists of red and white blood cells, platelets, and plasma (the goop in which everything sits). Antigens and various proteins float in the plasma and on red blood cells. An antigen is any substance that causes the immune system to produce antibodies to fight intruders.

The ABO grouping system refers to genetically-determined individual differences in the presence of two antigens (A and B), which stimulate the production of different antibodies. Type-O blood has both the antibodies produced in type-A and type-B, whereas type-AB has neither.

In 2004 researchers from University College London proposed that the presence of certain bacteria and intracellular viruses may have put evolutionary pressure on certain antigen-producing genetic mutations. In populations where viruses prevailed, gene O dominated. Those with bacteria-heavy environments found themselves more likely to have A or B type.

The major blood groups were not known until the early 1900s. Before then blood transfusions sometimes were fatal, because the different blood groups are incompatible. In 1940, experiments on Rhesus monkeys revealed additional antigen factors now known as positive or negative "Rh factors." This led to the types "O positive" or "AB negative." Since then, hundreds of other less-significant antigen differences have been identified, most of which do not lead to transfusion problems.