The natural occurrence and lifelong persistence of alloantibodies (isoagglutinins) to the ABO blood group antigens preclude all ABO-incompatible transfusions. For this reason transfusion became feasible only after the discovery of the ABO blood group system in 1900 using the erythrocyte agglutination technique. Clinically relevant alloantibodies to most other blood group antigens hardly occur before immunization by transfusions or pregnancies. Besides ABO, a few other antigens such as Rh D and Kell are always considered in transfusion practice in Caucasians, because of their prevalence and immunogenicity. The relative immunogenicity of antigens is determined by comparing the actual antibody frequency and the calculated frequency of the probability of exposure. The genes of almost all blood groups are cloned, allowing the molecular

Table 2 Infectious marker screening

Routinely tested in many countries

HIV-1/HIV-2 HIV-specific antibodies, sometimes p24 antigen

Hepatitis B virus HBsAg, sometimes HBc-specific antibodies

Hepatitis C virus HCV-specific antibodies

HTLV-1 HTLV-1 -specific antibodies

Syphilis Treponema pallidum-specific antibodies (TPHA) Alanine aminotransferase Enzymatic activity

Other pathogens and diseases commonly transmissible by blood

Cytomegalovirus, parvovirus B19, Epstein-Barr virus, yellow fever, malaria, Chagas disease, toxoplasmosis, leishmaniasis, onchocerciasis

HTLV-1, human T cell lymphoma virus.

characterization of the allelic diversity. Recently, all authenticated blood group antigens were revised according to their genetic basis and may fall into one of four classifications: systems, collections, low incidence antigens (700 series), and high incidence antigens (901 series). A numerical terminology was devised. Systems are genetically discrete from each other and are represented by antigens controlled by one or more very closely linked homologous gene loci. Collections comprise antigens, which are related but do not meet system status criteria. Low and high incidence antigens that do not belong to systems or collections are grouped into series. The antigen structure is generally constituted either by carbohydrates or by proteins (Table 3).

Since about 1960, efficient antibody detection and cross-matching (compatibility testing between donor and recipient) have been routinely applied using the antiglobulin (Coombs) test. A serologic incompatibility causing immediate or delayed hemolytic transfusion should rarely be a serious threat today. The recent introduction of matrix (gel) and solid phase tests increased the sensitivity of antierythrocyte antibody detection and holds promise in respect to standardization and automation.

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