By an enzyme (thrombin) Thrombin is formed from a blood-protein (prothrombin) by an activator (thrombokinase) liberated from injured tissues or from blood platelets (q.v); removal of free calcium from blood (as by adding oxalate or citrate) inhibits thrombin formation. In some Arthropoda clotting is by projection of thread-liked processes from blood corpuscles.

Blood Corpuscle Cell which circulates in blood. Particularly abundant in vertebrates.

Blood Film A very thin smear of blood, made on a slide, fixed dried, and stained. Used for examination of corpuscles and detection of blood parasites.

Blood Group Group of people bearing the same antigens on their red blood cells. There are many types into which an individual’s blood may be classified.

There are four main groups, whose blood cannot be mixed without clumping of their red blood cells; they are A, B, AB and O.

Proportions of the four groups within population are very different in different parts of the world. From point of view of blood transfusion, introduction of corpuscles containing agglutinogen for which recipient has agglutinin must be avoided. The converse is not so important.

Members of the population differ in other antigens carried on red blood cells (e.g. M and N antigens, forming M, MN, and N types); but since agglutinins for these are not normally present in plasma, they have not the same significance in transfusion though immunity to them may be acquired by repeated transfusion. Similar blood groups occur in other Primates.

Rh factor was first discovered in the erythrocytes of rhesus monkey in 1940 by Landsteiner and Wiener. 85% of human population has Rh factor in their erythrocytes (Rh+tive) while the rest lacks it (Rh-tive). Rh factor is very important for blood transfusion.