From school biology course we all know that everyone has some kind of a blood group: A (II), As (III), AB (IV) or O (I) - and only one. We are told that we can not have two sets of blood because the body will reject the wrong group, which falls into a transfusion. However, all this does not apply to blood chimeras.
Blood chimera - a person having two different types of blood and tissues, producing new blood cells of both types. Blood chimeras occur both in animals and in humans, but is very rare. All known human chimeras - twins. It is believed that somehow still in the uterus occur three very strange things. Blood is divided between the two fruits, they exchange the tissues that produce blood, and the normal response of the immune system - the rejection of another's blood - is suppressed.
If you have blood group A, it carries two proteins - antigen A and B. antibody antigen - a substance that encourages the body to produce antibodies. If your blood group, it carries the antigen and antibodies in A. If your blood group AB, it contains both antigens, but it does not have antibodies. Finally, if you have blood group O, it does not contain antigens, but there are both antibodies.
Thus, people with blood group A can safely receive blood group A or group O People with blood group B can receive blood group B or O People with blood group AB (universal recipient) can receive blood of any group, as they Blood is compatible with all the others. People with blood group O (universal donor) may give the rest of the blood, but can only receive the blood of his group.