Separation of anti-Ia (I-region associated antigens) from anti-H-2 antibodies in complex sera, by absorption on blood platelets. description of three new Ia specificities.

H-2 antigens are expressed in substantial amounts of murine blood platelets (for H-2 antigenic content 1 lymphocyte approximately 50 platelets) whereas Ia antigens are probably not expressed at all (minimal Ia antigenic content more than 35 times lower than for H-2). This property of blood platelets makes them very useful for the selective absorption of anti-H-2 antibodies from complex sera and for the preparation of specific anti-Ia antibodies from such sera. In 20 sera produced against the complete H-2 complex, 12 sera contained anti-Ia antibodies beside the expected anti-H-2 antibodies. In two sera, separation of the anti-Ia antibodies was easily obtained by absorption of the anti-H--2 antibodies on platelets. The analysis of one serum (C3H.Q X B10.D2) anti-C3H [(q X d) anti-k] showed that, in addition to be expected anti-H--2.23 and anti-Ia.2 antibodies, it contained at least three other Ia antibodies, separable by absorption on lymphocytes, which recognized three antigens--Ia.17, determined by the haplotypes k, f, s, r, j; Ia.18, determined by the haplotypes k, f, s; and Ia.19 determined by the haplotypes k and r. The genes are located in the I--A and/or I--B subregions of the H--2 complex.