Lec.3:Blood types

Blood Types and Transfusion
Losses of about 15 to 30 percent of blood lead to pallor and weakness. Losses of over 30 percent cause severe shock, which can be fatal. Whole blood transfusion are routinely given to replace substantial blood loss and to treat severe anemia or thrombocytopenia.


Antigenicity and Immune Reactions of Blood
When blood transfusions from one person to another were first attempted, immediate or delayed agglutination and hemolysis of the red blood cells often occurred, resulting in typical transfusion reactions that frequently led to death. Soon it was discovered that the bloods of different people have different antigenic and immune properties, so that antibodies in the plasma of one blood will react with antigens on the surfaces of the red cells of another blood type. If proper precautions are taken, one can determine ahead of time whether the antibodies and antigens present in the donor and recipient bloods will cause a transfusion reaction.
The plasma membrane of RBCs, like those of all body cells, bear genetically determined proteins (antigens), which identify each person as unique. An antigen is substance that the body recognizes as foreign; it stimulates the immune system to release antibodies or use other means to mount a defense against it. Most antigens are foreign proteins, such as those that are part of viruses or bacteria that have managed to invade the body. The person’s RBC proteins will be recognized as foreign if transfused into another person with differ RBC antigen. The recognizer are antibodies present in the plasma that attach to RBCs bearing surface antigens different from those on the patient’s (blood recipient’s) RBCs. Binding of the antibodies causes the RBCs to clump, a phenomenon called agglutination, which leads to the clogging of small blood vessels throughout the body. During the next few hours, the foreign RBCs are lysed (ruptured) and their hemoglobin is released into the blood stream. Although the transfused blood is unable to deliver the increased oxygen-carrying capacity hoped for, and some tissue areas may be deprived of blood, the most devastating consequence of severe transfusion reactions is that the freed hemoglobin molecules may block the kidney tubules and cause kidney failure.