Platelets, also called thrombocytes, are very numerous tiny cells. They are in fact more akin to cell fragments than entire cells. There are about 300,000 in each cubic millemetre of blood. Platelets originate from platelet-producing cells in the bone marrow, called megakaryocytes, and are necessary for the blood to clot. This is clearly vital after an injury but is equally important in resisting the stresses and strains of everyday life. Without adequate numbers of platelets, blood would ooze from the linings of the mouth, nose, gut, and bladder, and also into tissues such as the skin. Platelets have the potential to stick together to form bodies (agglutinins) against the A, B or AB antigens. For example, a person with blood group A has anti-B agglutinins. When a blood tranfu-sion is given, the recipient and donor must have matching blood.

The red blood cells of people with the blood groups A, B or AB contain antigens which cause coagulation with the serum (blood plasma without fibrinogens) of other groups. This is because of the presence in the serum ofanto- clumps and this property is controlled by chemical factors, some of which promote clotting and some that oppose it. There are 12 clotting factors, all essential to prevent bleeding.

Following a cut or similar injury, tissue damage and exposure to air activate the clotting process. The clotting factors react with each other in turn, a phenomenon known as a cascade reaction, and the end result is production of the protein fibrin. Fibrin forms a network across the site of injury and platelets stick to this mesh, thereby sealing the break. The resulting mesh of fibrin, platelets and blood is the typical blood clot we all recognize.

Lack of a clotting factor impairs this process and congenital absence of one or more factors does occasionally occur. Haemophilia is the best-known of these conditions; it is caused by the absence of factor VIII. Reduction of the ability of the blood to clot may also be the result of drugs. Overenthusiastic use of aspirin, for example, is a common cause of gastrointestinal bleeding and anaemia. Under normal conditions blood does not clot in the circulation, but sometimes the control mechanisms are upset and a blood clot forms within a vessel. This clot is known as a thrombus. If a thrombus develops in one of the coronary arteries which supply the heart muscle with oxygen and nutrients, the result is a coronary thrombosis – commonly known as a heart attack. A clot obstructing an artery supplying blood to the brain can cause a stroke.