Human blood grouping is mainly based on the antigens present on their RBC (red blood cells) and in plasma into various groups but the main antigens are namely A/B and Rh antigens.
Depending on A/B antigens humans are grouped (ABO Grouping) into
A group: those having A named antigen on their RBC.
B group: those having B Antigen on their RBC.
AB group : Those having both A B antigens on their RBC.
O group: those having no antigens on their RBC.
Depending on the presence of Rh antigens they are grouped into Rh positive or negative group.
So if the person has both A and Rh antigens then he will be A positive blood group. But having only A antigen but not Rh Antigen then he will be A negative group.
Extra Information:
In addiition to these there is also the HH group. The peculiarity is that they do not express the H antigen. As a result they cannot form A antigens or B antigens on their red blood cells. Thus they can donate blood to anybody with ABO grouping but can receive blood only from Bombay blood group people.
