Congenic anti-Ia antisera were used to bind radiolabelled Ia antigens from cells of various strains of mice of known H-2 haplotype. The results indicate that Ia antigens are proteins of molecular weight 30,000 to 35,000 daltons. The Ia antigens are distinct from known H-2 antigens as judged by independent immunoprecipitation as well as by molecular weight. Ia antigens are synthesized by, and are present on the surface of lymphoid cells as evidenced by incorporation studies using3H-leucine and enzymatic radioiodination of cells, respectively. Tissue distribution of cell surface Ia suggests that Ia antigens are on B cells. Ia antigens were detected in the incubation media of3H-leucine labeled splenocytes suggesting that antigens may be secreted.
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