There are four types of blood in humans. Type A, B, AB, and O. The type is due to the presence of antigens(proteins) found in the blood. Type A contains antigen A. Type B contains antigen B. Type AB contains both antigens. Type O contains neither antigen A nor B. Blood type is determined by one gene that each parent donates to their offspring. In terms of blood transfusions, a person always can get a donation of their own blood type or type O. Type O is the universal donor because their blood lacks antigen A and antigen B. If someone receives blood with the wrong antigen for their blood type, an immune response occurs and clumping will occur. However, this immune response won't occur with type O blood, so it can be donated to anyone. Type AB is called the universal recipient. That is because this type contains both antigen A and B and any type will be familar to their system, so no immune response occurs. Not only are there four blood types, the blood can be Rh positive or Rh negative. Rh positive blood contains another blood protein, the Rh factor and Rh negative blood does not have this protein present. Most people are Rh positive, since this is a dominant trait. So, a person not only is categorized by blood type, but, also, whether that blood type is postivie or negative.