Whole blood or plasma is collected from volunteer donors. Over 90% of the donated blood is separated to allow use of individual cell components and plasma from which specific blood products can be manufactured (Fig. 48.1).

In the UK, blood donors are healthy volunteers, aged 17–70 years, who are not on medication, have had no serious previous illnesses and are at low risk for transmitting infectious agents. Those who have received blood product transfusions, drug abusers, haemophiliacs, selected individuals who have recently travelled outside Europe or lived in Africa – where malaria or AIDS may be endemic – and their sexual partners are excluded. Donors are screened for anaemia and they donate two to three times each year.

Donated blood is routinely tested in the UK:

For hepatitis B and C, HIV 1 and 2, HTLV I and II, Treponema pallidum;
Serologically to determine the blood group (A, B or O) and Rh C, D and E type;
Selective testing for antibodies to cytomegalovirus (CMV) is used to identify donations which are CMV-negative and thus suitable for certain patients; and
In other parts of the world, testing is performed for trypanosomiasis (Chagas disease) or West Nile virus.

Blood grouping and compatibility testing

Red cells

Red cells have surface antigens which are glycoproteins or glycolipids (Table 48.1). Individuals lacking a red cell antigen may make alloantibodies (antibodies in one individual reacting to cells of another individual) if exposed to it by transfusion or by transfer of fetal red cells across the placenta in pregnancy. Antibodies to ABO antigens occur naturally, are IgM and complete (detectable by incubation of red cells with antibody in saline at room temperature). Antibodies to other red cell antigens appear only after sensitization. They are usually IgG and incomplete, detected by special techniques, e.g. enzyme-treated red cells, addition of albumin to the reaction mixture or the indirect antiglobulin reaction. Alloantibodies can cause:

intravascular (e.g. ABO incompatibility) or extravascular (e.g. Rh incompatibility) haemolysis of donor red cells in the recipient; and
haemolytic disease of the fetus and newborn because of transplacental passage.

Table 48.1 Red cell antigens and antibodies. Incidence in UK individuals given in brackets

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