Like HowStuffWorks on Facebook!

How Blood Works

Ensuring a Safe Blood Supply
There are many tests that are performed on blood to ensure its safety. These tests include checking for:
  • Hepatitis B surface antigen
  • Hepatitis B core antibody
  • Hepatitis C antibody
  • HIV-1, HIV-2 antibodies
  • HIV-1 p24 antigen
  • HTLV-1, HTLV-2 antibodies
  • Syphilis

If any of these tests are positive, the blood is discarded. As of 1996, the risk of getting HIV from a single blood transfusion was 1 in 676,000 units of blood, the risk of developing Hepatitis B was 1 in 66,000 units and the risk of getting Hepatitis C was 1 in 100,000 units. However, newer testing may decrease the risk of Hepatitis C to between 1 in 500,000 and 1 in 1,000,000.

When blood is transfused into a patient, the blood type must be determined so that a transfusion reaction does not occur.

A reaction occurs when the antigens on the RBCs of the donor blood react with the antibodies present in the recipient’s plasma. In other words, if donor blood of type A (contains A antigens) is given to someone with type B blood (they have anti-type A antibodies in their blood), then a transfusion reaction will occur.

The opposite does not occur. It is unusual for the antibodies in the plasma of the donated blood to react to the antigens on the recipients RBCs because very little plasma is transfused and it gets diluted to a level too low to cause a reaction.

When a transfusion reaction occurs, an antibody attaches to antigens on several RBCs. This causes them to clump together and plug up blood vessels. Then they are destroyed by the body (called hemolysis), releasing hemoglobin from the RBCs into the blood. Hemoglobin is broken down into bilirubin, which can cause jaundice. These events occur in hemolytic disease of the newborn (mentioned previously).

When an emergency blood transfusion is necessary and the recipient's blood type is unknown, anyone can get type O- blood transfused since type O- blood has no antigen on its surface that could react with antibodies in the recipient’s plasma. Therefore, someone with type O- blood is called a universal donor. Someone with type AB blood is called a universal recipient because they have no antibodies that could react with donated blood.

For more information on blood and related topics, check out the links on the next page.

More to Explore