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Blood groups
Blood group, classification of blood based on inherited differences (polymorphisms) in antigens on the surfaces of the red blood cells (erythrocytes). Inherited differences of white blood cells (leukocytes), platelets (thrombocytes), and plasma proteins also constitute blood groups.
English physician William Harvey announced his observations on the circulation of the blood in 1616 and published his famous monograph titled Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (The Anatomical Exercises Concerning the Motion of the Heart and Blood in Animals) in 1628. His discovery, that blood circulates around the body in a closed system, was an essential prerequisite of the concept of transfusing blood from one animal to another of the same or different species. In England, experiments on the transfusion of blood were pioneered in dogs in 1665 by physician Richard Lower. In November 1667 Lower transfused the blood of a lamb into a man. Meanwhile, in France, Jean-Baptiste Denis, court physician to King Louis XIV, had also been transfusing lambs’ blood into human subjects and described what is probably the first recorded account of the signs and symptoms of a hemolytic transfusion reaction. Denis was arrested after a fatality, and the procedure of transfusing the blood of other animals into humans was prohibited, by an act of the Chamber of Deputies in 1668, unless sanctioned by the Faculty of Medicine of Paris. Ten years later, in 1678, the British Parliament also prohibited transfusions. Little advance was made in the next 150 years.
Antibodies and antigens
Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma. blood group is identified by antibodies and antigens in the blood. 
Antibodies are proteins found in plasma. They’re part of body’s natural defences. They recognise foreign substances, such as germs, and alert immune system, which destroys them. Antigens are protein molecules found on the surface of red blood cells.
ABO system
When it comes to blood type, you might inherit an A antigen from one parent and a B antigen from the other, resulting in the AB blood type. You could also get B antigens from both parents, giving you a BB, or a B, blood type. Type O, on the other hand, doesn’t contain any antigens and has no effect on A and B blood types. This means that if you inherit an O from your mother and an A from your father, for example, your blood type would be A. It’s also possible that two people with type A or type B blood could have a baby with type O blood if they carry the O antigen. For example, parents with AO blood could each pass the O antigen on to their child, creating OO (or simply O) blood. There are six of these combinations (AA, AB, BB, AO, BO, OO), which are called genotypes. The four blood types (A, B, AB, and O) stem from these genotypes.
Rh factor
Blood is also typed according to something called the Rh factor. This is another antigen found on red blood cells. If the cells have the antigen, they’re considered Rh-positive. If they don’t have it, they’re considered Rh-negative. Depending on whether the Rh antigen is present, each blood type is assigned a positive or negative symbol.
Significance of blood types
Your immune system naturally contains protective substances called antibodies. These help to fight off any material that your immune system doesn’t recognize. Usually, they attack viruses and bacteria.
However, antibodies can also attack antigens that aren’t present in your natural blood type. For example, if you have type B blood that’s mixed with type A blood during a transfusion, your antibodies will work to destroy the A antigens. This can have life-threatening results, which is why medical centers around the world have strict procedures in place to keep this from happening.
Keep in mind that blood types don’t always need to be an exact match to be compatible. For example, AB blood has both the A and B antigen, so a person with this type of blood can receive either type A or type B blood. Everyone can receive type O blood because it doesn’t contain any antigens. This is why people with type O blood are considered “universal donors.” However, people with type O blood can only receive type O blood.
When it comes to the Rh factor, people with Rh-positive blood can receive either Rh-positive or Rh-negative blood, while people with Rh-negative blood can only receive Rh-negative blood. In some cases, a woman with Rh-negative blood can carry a child with Rh-positive blood, resulting in a dangerous condition called Rh incompatibility.
Blood group test
To work out your blood group, your red cells are mixed with different antibody solutions. If, for example, the solution contains anti-B antibodies and you have B antigens on your cells (you’re blood group B), it will clump together.
If the blood doesn’t react to any of the anti-A or anti-B antibodies, it’s blood group O. A series of tests with different types of antibody can be used to identify your blood group. If you have a blood transfusion – where blood is taken from one person and given to another – your blood will be tested against a sample of donor cells that contain ABO and RhD antigens. If there’s no reaction, donor blood with the same ABO and RhD type can be used.
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