When asked, many people do not know their blood type, but a person’s blood type is very important in a medical emergency that might require a blood transfusion or an organ transplant. The study of blood is a particular specialty in medicine known as hematology. There are 29 different blood group systems, each represented by specific elements found on the surface of red blood cells. The most common blood group systems are ABO and Rhesus.

A person’s blood type becomes a crucial matter when a blood transfusion is necessary. If an individual is transfused with the wrong blood type, a reaction that severely compromises the immune system can result. There are different levels at which a mismatched transfusion can occur, from minor to major incompatibility. Minor factors may lead to very minor problems, but a major incompatibility of blood type during a transfusion can be serious.

Blood type is so important during a transfusion that donors giving blood to a blood bank must meet certain requirements. A blood transfusion is closely monitored, often by a hematologist, and the ordering of blood products in the United States is regulated by the Food and Drug Administration.

In the case of organ transplants, the tissue of the donor organ must match the recipient’s blood type. A mismatch can cause rejection, and therefore, without a positive match, the transplant will not be conducted. Individuals placed on a transplant waiting list have their blood type thoroughly tested before being placed on the list.

Blood type is also significant for a pregnant woman and her fetus. A woman’s blood type is usually determined during routine blood work early in her pregnancy. In cases in which the mother’s blood type does not match the baby’s, which are frequent, it is possible for the mother’s body to develop antibodies to defend against the red blood cells of the fetus. If these antibodies pass through the placenta, they can cause the baby to develop a blood disorder or disease. Though most complications of this situation range from mild to moderate, severe or even fatal reactions can occur.