ABO blood grouping is determined by the correlation of visible clumping in mixtures of plasma, the liquid component of blood, and the red cells, which carry oxygen.
The Rhesus system, first discovered in 1939, relies on the presence of antigens – the D antigen in particular. Antigens are molecules that bind to an antibody. A person either has or hasn’t got the Rh factor (D antigen) on the surface of their red cells, indicating a positive (Rh+) or negative (Rh-).