Blood is composed of cells suspended in a liquid. These cells - red cells, white cells, and platelets -account for 45% of the volume of blood. The remaining liquid portion is the plasma, from which therapeutic fractions and derivatives are made. Blood is the most vital thing for human organism. It helps for the physical growth in humans. If there is no blood in human body many important parts of the body will stop functioning.

   Red cells in the blood transport oxygen to body cell and remove carbondioxide.

   White cells (leukocytes) are the protective cells in the blood stream. They attack bacteria by squeezing through capillary walls to reach the area of infection where they destroy bacteria. White blood cells are also made in the bone marrow, and are produced at twice the rate of red blood cells. It plays a important role in protecting us from so many diseases and enable us to live a healthy life .

   On the surface of every RBC are to be found specific glycoproteins and glycolipids that act as antigens. RBC antigens are termed isoantigens or agglutinogens. The types of isoantigens present allow blood to be categorised into different blood groups. Over100 isoantigens have been identified, defining more than two dozen different blood groups. Although in cross-typing blood, all bloodgroups need to be accounted for, two blood groups are of particular importance. These are the ABO blood group and the Rhesus blood group.

ABO Blood group: Two isoantigens are involved in the ABO blood group. These are termed antigen A and antigen B. Various combinations of the presence or absence of A and B antigens produce four different blood types, known as type A, type B, type AB and type O.
Rhesus blood group: The Rhesus system was discovered by Landsteiner in 1941. In his search for blood group antigens he injected RBCs from rhesus monkeys into rabbits and guinea pigs, so producing anti-rhesus antibodies which agglutinated the monkey RBCs. These antibodies were found to agglutinate the blood from 85% of Caucasians, who were thereby designated Rhesus-positive (Rh+). The 15% of the populations whose RBCs failed to agglutinate were termed Rhesus-negative (Rh-).