Blood is very important; it carries oxygen and nutrients to all parts of the body. Blood is made up of white and red blood cells, platelets, and plasma. Platelets are responsible for clotting, and plasma is like the "river" in which blood cells "swim" in. White blood cells fight infections and red blood cells transport oxygen.

In 1901 Karl Landsteiner discovered that there were different types of blood. He decided that once blood was classified, then transfusions would be successful. Blood typing is based on the presence of certain proteins called antigens. Antigens are capable of stimulating a response from the immune system. The ABO system of blood typing is based on the presence of A and B antigens.

Human bodies have antibodies in their plasma against the blood antigens they do not have. For example, type A people have A antigens on their red blood cells, and Anti-B antibodies in their plasma. Type AB people have both A and B antigens on their red blood cells, and therefore no antibodies in their plasma. Type O people have no antigens on their red blood cells, so they must have both Anti-A and Anti-B antibodies in their plasma. When antigens and antibodies interact with each other, the red blood cells clump together. This blood clotting is deadly.

When blood is tested, an antibody serum is added to the unknown blood to detect any antigens. If any are detected, then clotting occurs. For example if the anti-A antibody serum is added to the blood, and it clots, then the blood is type A. Blood type AB clots when either A or B antibody serum is added, and type O will never clot.

Recently there has been a shortage of the world's blood supply. Japan has been doing experiments to develop artificial blood that will work in human bodies. However, tests are inconclusive so far.