The immune response protects the body from harmful substances. Foreign substances called antigens cause many immune responses. They trigger the production of proteins, called antibodies, which attach to antigens and help destroy them. They also trigger a type of white blood cell called lymphocytes, which recognize a certain antigen and destroy it. The immune system normally can tell its own blood cells from other cells. On the surface of red blood cells are proteins that the body sees as antigens. In 1900, a German disease expert (pathologist), Karl Landsteiner, found 2 of these proteins, which he called A and B. Blood is arranged by these antigens into blood types A, B, AB (contains both antigens), and O (contains neither antigen). The liquid part of blood (plasma) has antibodies against the opposite antigen. A person with Type A blood, for example, has antibodies against the B antigen. In 1940, Dr. Landsteiner found another group of antigens. They were named Rhesus factors (Rh factors) because they were discovered during experiments on Rhesus monkeys. People with Rhesus factors in their blood are called "Rh positive." Persons without the factors are called "Rh negative." Rh negative people form antibodies against the Rh factor if they are exposed to Rh positive blood.This can be a problem in an Rh negative mother who is pregnant with an Rh positive baby. There are other antigens too, besides ABO and Rh antigens. Having antibodies against blood antigens makes blood groups compatible or incompatible. Tranfusing blood between compatible groups (such as O+ to O+) usually causes no problem. Blood transfusion between incompatible groups (such as A+ to O-) causes an immune response against the cells carrying the antigen. This can lead to a transfusion reaction. The immune system attacks the donated blood cells, causing them to burst. This may cause serious symptoms, including kidney failure and shock. Antigens also occur in other parts of blood, including white blood cells, a type of blood cell called platelets, and plasma proteins. These parts of the blood also cause a similar type of transfusion reaction. Or, antibodies in the transfused blood can bind to the patient's blood cells, causing a reaction. Today, all blood is carefully screened. Modern lab methods and many checks have helped make transfusion reactions very rare. | 
Hematology
