There are different ways to classify blood. The two major forms of classification include

the ABO system  and rhesus (Rh) type system.

The ABO group

The four different blood groups are A, B, AB and O.

A person's blood group is determined by a pair of genes, one each inherited from their

mother and father.

Each blood group is identified by its own set of complicated chemical substances  called

antigens  located on the surfaces of red blood cells. When a person needs a blood

transfusion, it is important that the donated blood matches  their particular blood group.

mismatch can cause serious complications

A person's Rhesus type is also determined by a pair of genes, each one inherited from

one parent.

Blood is either Rh-positive or Rh-negative, depending on whether or not certain

molecules are present. A person who is Rh-negative will experience a severe immune

 system reaction if Rh-positive blood gets into their bloodstream

This can happen during childbirth, if an Rh-negative woman gives birth to an Rh-positive

baby.

If blood cells from the baby travel across the placenta, the woman's immune system will

regard the Rh-positive cells as a great threat.

Specialized white blood cells will make antibodies designed to kill Rh-positive blood cells.

flood her child with antibodies

These antibodies then destroy the baby's red blood cells. If left untreated,  can result in

severe anemia or even death.

Rhesus disease is now rare, since Rh-negative mothers who give birth to Rh-positive

babies are immunized within 72 hours of giving birth.

The immunoglobulin preparation works by killing the baby's red blood cells inside the

mother's bloodstream before her immune system has time to react.

Blood transfusion

It is the transfer of blood from one person to another. The donated blood must match

 the recipient's blood type, or complications will occur