Blood transfusion is the taking of blood or blood-based products from one individual and inserting them into the circulatory system of another. It can be considered a form of organ transplant. It is performed as a treatment for various medical conditions, such as massive blood loss due to trauma, surgery, shock and other conditions in which tissues of the body are not adequately oxygenated, where carbon dioxide or other toxic materials are not being effectively removed, and where the red cell producing mechanism (or some other normal and essential component) fails.
Great care is taken to ensure that the recipient's immune system is compatible with the foreign blood, and also that the foreign blood is compatible with the donor's immune system. In addition to the familliar human blood type (A, B, AB and O) classifications and the Rhesus factor classification (positive or negative), today a number of other tissue type factors are also known to determine histocompatibility, to one degree or other. These tissue type factors become increasingly important in people who receive many blood transfusions as their bodies develop increasing resistance to blood from other people.
There is increased awareness that a number of diseases (such as AIDS, human T-Cell lymphotropic virus which causes a form of leukemia, syphilis, hepatitis B and hepatitis C) can be passed from the blood donor to the transfusion recipient, and tight human blood transfusion standards are the rule in all developed countries. Standards include screening for potential risk factors and health problems through such steps as the taking of blood pressure, determining if the donor has enough hemoglobin (the iron-containing protein in blood cells which enables efficient transport of oxygen), and answering a set of standard oral and written questions, as well as testing donated units for these infections. The lack of such standards in places like rural China where desperate villagers donated plasma for money and had others' red blood cells reinjected has produced entire villages infected with the AIDS virus.
Blood substitutes that will avoid this risk, address the chronic blood donor shortage and address some religious objections of groups such as the Jehovah's Witnesses, are currently in the clinical evaluation stage.
The entire process of donating blood takes about 45 minutes. About 450 ml is taken. Afterwards, the consumption of moderate amounts of non-alcoholic liquid refreshments is encouraged, as this assists with fluid replacement which occurs within a few hours. Other components are replaced by the donor's own body within a few days or weeks.
Depending on the needs of the community at any one time sometimes only parts of the blood are taken as a donation. Blood is made up mostly of red blood cells, platelets - essential in the normal clotting mechanism, and plasma. The latter two can be donated seperately in a process called ~apherisis. Blood is often separated into components after being donated. These components also include albumin protein used to treat burns, clotting factor concentrates used to treat hemophilia, cryoprecipitate, and immunoglobulin antibodies for immunological disorders.
In cases where whole blood is donated, there is no transfusion-related risk of illness for the blood donor, aside from the minuscule chance of infection or perhaps of localized injury to the donor site. There is some risk to the donor in cases where the donor donates plasma and has red blood cells reinfused. This risk is nonexistent in cases where proper sterilization procedures are followed, but has caused public health disasters in cases such as AIDS in central China where they were not often regulated. The best-known examples of safe apheresis donation are the United States' blood plasma collection centers, maintained by pharmaceutical companies, using paid donors up to twice-weekly.
Donations are usually anonymous to the recipient, but products in a blood bank are always individually traceable through the whole cycle of donation, testing, separation into components, storage, administration to the recipient, and any resultant disease.
It is interesting to note that veterinarians occasionally administer transfusions to animals in need of this treatment. Various species require different levels of testing to ensure a compatible match. Cats have 3 blood types, cattle have 11, dogs have a dozen, pigs 16 and horses have 34.
The rare and experimental practice of inter-species blood transfusions is a form of xenograft.
Research into blood substitutes which could make blood transfusions more readily available in emergency medicine and even in pre-hospital EMS care continues. If successful such a blood substitute could save many lives, particularly in traumas where massive blood loss results.
While it is not strictly a type blood transfusion, the formal practice of sealing a long term relationship through the co-mingling of blood to become blood brothers should be mentioned. This is a social ritual or custom, with no medical justification or indication. The practice carries with it a slight risk of blood born illness, infection or scarring.