The sera of 172 HL-A typed subjects irrirnunized mostly by multiple transfusions were studied and the specificities of the produced H L A antibodies were determined. The probability of non-response against an antigen cross reacting with an antigen of the immunized subject was calculated by the use of Bayes' method. A lower immunogenicity within the main cross reacting groups (CREG) was confirmed: [HI,-A2, W281, [HL-A3,1,11], connected to [HGAlO, W19 (=W29,30,31,32)], [W5, HL-AS, W15,W21], [HL-.47,WlO,W22]. Some antigens were not connected to any CREG: HL-A9,12,13, W27. Others were insufficiently studied in the material to allow definite conclusions: W 18,W 14,HL-A8. An asymmetry of the phenomenon i.e. non reciprocal inhibition of antibody production for two antigens was observed often.The comparison of one part of the rriaterial studied with the platelet complement fixation test, to another part studied with the lymphocytotoxicity technique suggested that the specific inhibition of antibody production was not complete but that such a situation only decreased the frequency and intensity of the response against the antigen.A study of the co-occurrence of various HL-A specificities in the immune sera lead to a similar but somewhat broader description of the GREG.

Screening for anti‐HLA‐DR sera was performed by complement fixation on PHA stimulated peripheral blood lymphocytes (PHA‐CF), or cultured B lymphoid cell lines. Out of 1,350 sera from multiparous women, multitransfused patients, and patients transfused during extra‐corporal circulation (ECC), 219 contained anti‐HLA‐DR activity (16.2%). Anti‐HLA‐DR antibodies developed after ECC were often high titered (1:10 to 1:100). In half of these sera anti‐HLA‐A, B antibodies were weak or absent, making it possible to use them as anti‐HLA‐DR reagents without platelet absorption. Of the 219 positive sera 51 contained defined anti‐DR antibodies (20 monospecific and 31 bi‐or multispecific). The 13 best sera recognized DR1 to DR7 specificities with r values from 0.83 to 1. Twenty‐four sera selected by CF were also studied by lymphocytotoxicity technique against peripheral blood B lymphocytes (B‐LCT). Both PHA‐CF and B‐LCT techniques gave similar results, detecting the same specificities and showing comparable sensitivity. The advantages of CF are: easy storage of target cells at −80°C or +4°C, and fast reading. For these reasons PHA‐CF or CF on cultured B lymphoid cell lines can be proposed for large scale screening of anti‐HLA‐DR sera. The sera thus screened can be used for HLA‐DR typing either by PHA‐CF or B‐LCT.