There are about one billion red blood cells in two or three drops of blood.
Blood groups are complex chemical systems found on the surface of blood cells. The two main blood group systems in transfusion practice are the ABO system and the Rh(D) type. Identification of the correct blood group is important to prevent reaction following transfusion.
Within the ABO system, people can be one of four groups - O, A, B or AB, while in the Rh(D) system they can be either Rh(D) positive or Rh(D) negative. Each system is inherited independently of the other. As a result, there are eight main blood groups.
The Givers/Receivers chart shows the compatibility of different blood groups. For example, an A negative patient may receive blood from an O negative or an A negative donor.
Play an online game of blood transfusions (external link) on the Nobel Prize website to learn more about blood typing and transfusions.
Also see the Education section for learning aids to use in the classroom to educate students about blood.
O is for Extra Ordinary Leaflet (pdf, 776 KB)