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 on murine blood platelets (for H‐2 antigenic content 1 lymphocyte ∼ 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 themvery 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 × B10.D2) anti‐C3H [(q × d) anti‐k] showed that, in addition to the 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.