Blood transfusion is the process of passing blood from a donor into the circulatory system of the recipient. The reason for the procedure may be severe anaemia or the loss of a great amount of blood through injury. Blood is usually taken from an arm vein of a healthy donor by inserting a needle into the vein and drawing blood into a receptacle containing a little sodium citrate to prevent clotting. Blood can be stored at about 4°C for several weeks. It is essential that the donor’s type of blood is ‘matched’ to that of the recipient. There are substances in everyone’s blood, known as antibodies, that react against foreign proteins (antigens), which include foreign blood cells. Fortunately, many individuals share the same types of blood cells and antigens and thus have ‘compatible’ blood.
Blood is classified into four main groups with different degrees of compatibility. Group A can be safely mixed with group A, but it causes clotting if mixed with group B. Group B can be mixed with group B but not with group A. Group AB can safely receive blood from groups A or B, but cannot be given to either. Group O blood (the universal donor) can be given to A, B or AB but, if you are group O, then you can only receive group O blood.
There is another antibody system known as the Rhesus factor, which exists in two forms: Rhesus positive and Rhesus negative. Both donor and recipient must belong to the same Rhesus group in addition to a compatible ABO group. Whole blood may be transfused in cases when there has been great blood-loss, such as in a road traffic accident, but blood is more often separated into its constituents before it is used. Blood can be divided into cells and the remaining fluid, called plasma. The plasma is separated from the cells by centrifugatton (spinning it very fast in a special machine), and then separated into further components by various techniques. In this way, a single donation of blood can be used to supply the different needs of a number of patients. Separating the blood components also means that smaller volumes have to be transfused than whole blood, which helps to avoid overloading the circulation of the patient.
Giving a transfusion
A donor usually gives about 500ml of blood in one session (an average person has 5 litres of blood so taking this amount generally causes no problems); but the quantity given to the recipient will vary with the circumstances.
Viral hepatitis can be transmitted in whole blood, plasma, or plasma fractions. Malaria can also be passed from donor to recipient via a blood transfusion. Syphilis can be carried in fresh blood, but is killed at low temperatures. The virus responsible for acquired immune deficiency syndrome (AIDS) is known to have been transmitted to haemophiliacs, probably in factor VIII concentrates, for which the blood of hundreds of donors contributes to a single transfusion. Factor VIII has recently been artificially synthesized, and this will provide a risk-free future treatment for haemophiliacs. Some people, such as Jehova’s witnesses, will not accept blood transfusions.
Artificial blood, a fluid containing electrolytes and other substances, has been used to help them. It breaks down rather quickly in the body, but can act as a stop-gap while the patient gains strength.
Compatible with the patient’s own, it is put into a container hung on a stand above the patient. The blood is then allowed to flow through a filter into a plastic tube at the end of which is a needle inserted into one of the patient’s veins. The rate of flow and the quantity received is monitored carefully by a doctor. The process, if somewhat time-consuming, is painless and rarely produces any complications. The number of volunteer donors has increased with public awareness of the need for well-stocked blood banks. The body quickly replaces the blood, but as a precaution, donors are advised to give blood only at two-month intervals.
Blood components and their uses
Packed red blood cells, minus plasma, may be given to severely anaemic patients. Only this component of blood is transfused because an anaemic patient lacks only haemoglobin, which is present in the red blood cells.
Platelets play an important part in blood clotting, and transfusions of platelets may be given to people with platelet deficiency who are at risk of prolonged bleeding if they are injured.
There are some occasions when whole plasma is needed, such as in an emergency when no whole blood (with cells) is available. Plasma can be used fresh, but it is just as useful as a preserved resource -either frozen or dried. As well as its use in restoring blood volume (thus preventing a fall in blood pressure) in cases of shock or haemorrhage, plasma contains all the clotting factors of whole blood and can therefore be given to stop prolonged bleeding. For a normal blood clotting reaction, platelets are needed as well as several chemical substances (factors). The antihaemophiliac factor (factor VIII) can be separated from plasma and used to treat haemophilia patients who suffer blood clotting problems because of factor VIII deficiency. This is especially useful as a precautionary measure for haemophiliacs about to undergo dental or other surgery. Albumin-rich components of plasma are useful in the case of people with a low plasma protein level (hypo-proteinaemia) as a result of starvation, gastrointestinal disease, liver disease or kidney disease. Children suffering from acute leukaemia and newborn babies suffering from blood diseases are sometimes given an exchange transfusion. This entails removing most or all of the child’s or baby’s blood while at the same time replacing it with a transfusion.
Risks of transfusion
There are some hazards associated with blood transfusion, besides those caused by a transfusion of non-compatible blood (which fortunately seldom happens). A few diseases, for example, can be transmitted by infected blood. Blood transfusion organizations take great care to screen potential donors for v