Positive or Negative Blood

When people are told there blood type they are usually told a letter, based on the ABO blood grouping system, and either a positive or negative, based on the Rh factor blood grouping system. If you have positive blood then you have an Rh antigen in their blood plasma. Those who have negative blood do not have the Rh antigen present in their blood plasma, however, they can obtain the Rh antigen by being the receiver in a transfusion involving positive blood. A person with positive blood can receive blood from a person with negative blood with no problems because they already have the Rh antigen.

For more information see: http://nobelprize.org/educational_games/medicine/landsteiner/readmore.html