Blood type is defined by the antigens (proteins that are coded to stimulate an immune response when a foreign substance is found) located on the surface of the red blood cells. There are two different antigens available to be on said surface. They are antigen A and antigen B. These antigens make certain blood types incompatible with each other because antigens will reject foreign substances with the wrong antigens. These two factors yield four different blood type combinations. Each blood type has requirements for recieving blood for a blood transfusion and also for donating blood.

Blood with no antigens at all is known as Blood Type O. This is the universal donor, and everyone can recieve this blood for a transfusion. Blood type O people can only recieve Type O transfusions.

Blood with both antigens is called Blood Type AB. This is the universal reciever. Only people with type AB blood can recieve AB blood. But, AB people can recieve any type of blood, hence universal reciever.

Type A blood people (Blood with just the A antigen) can only recieve Type A blood and Type O blood.

Blood Type B people can only recieve Type B blood and of course Type O.

There is another factor to blood types called the Rh factor. This is another membranal protein found on the surface of the Red blood cells. It is named after the Rhesus monkey in which it was first discovered. A red blood cell can simply either have the Rh protein, or just not. Thus, the notation for the Rh factor is Rh+ and Rh- (positive and negative) If an Rh+ is mixed with an Rh-, conglomeration will result (clumping) and that can sometimes cause serious problems. Rh factor is the reason why it is often required to test blood before marriage. This is because if an Rh+ father has a child with an Rh- mother and the child inherits the Rh+ trait, the baby's Rh+ blood can be transfered to the mother through the placenta during childbirth. This in itself does not pose an immediate problem, seeing that an adult can typically survive an Rh protein mixture. After such a blood transfer, the mother will often adopt the Rh protein to become somewhat of an Rh hybrid. If the couple has another child, and it turns out being Rh-, the mothers Rh hybrid blood may pass into the baby during childbirth and result in conglomeration for the baby. Sometimes, this blood clumping can prove fatal for the newborn. Thats about it for blood types

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