What Is Blood Type

• Blood type is based on whether a person's blood has certain antigens, molecules on the surface of the red blood cells. The two antigens used are called A and B. Blood with only A antigens is type A; with only B antigens, type B. Blood having both antigens is type AB. Blood with neither antigen is type O. These eight types are classified again using a third antigen. Blood with it is Rh positive; blood lacking it is Rh negative.
  
  

Significance

• Blood type becomes crucial in transfusions and organ transplants; only certain bloods should be mixed. If the blood types are incompatible, the patient's body will destroy the new blood's cells, leading to physical complications.
  
  

Common and Uncommon Types

• The most common blood type is O positive; 38 percent of people have it. The next most common is A positive, in 34 percent. The more uncommon types are B positive (9 percent), O negative (7 percent), A negative (6 percent), and AB positive (3 percent). B negative (2 percent) and AB negative (1% percent) are the rarest.
  
  

Interesting Fact

• A National Cancer Institute study released in August 2009 found a link between blood type and the development of pancreatic cancer.
  
  

All Types Needed

• Since only certain types of blood can be used together, regular blood donation helps ensure that physicians have all types available.