These red blood cells are also called erythrocytes or red blood cells. They contain hemoglobin, a protein that binds oxygen and gives blood its red color. The membranes of red blood cells carry antigens that determine the different blood groups. There are four blood groups: O, A, B, and AB. The Rhesus system is linked to the presence of a substance called D, which specifies the blood combination: there are therefore eight Rhesus groups/combinations.
