Rh is an antigen. If you have the antigen, your blood type is A+, B+, AB+, or O+. If you don't have the antigen, your blood type is A-, B-, AB-, or O-. People WITH the Rh antigen, can receive from/give to people with Rh+ or Rh-. People WITHOUT the Rh antigen, can ONLY receive from/give to people with Rh- blood. (meaning people without the antigen) If Rh- and Rh+ blood comes in contact, the Rh- will produce antibodies towards Rh+, and at the second contact with the blood, will cause agglutination.