Expression of la Antigens on T and B Cells and their Relationship to Immune‐Response Functions

The experiments described depend a great deal on a firm serologic and genentic definition of the strains used. In a number of cases, it can not be absolutely stated that all of the reactivity in the anti Ia serum is associated with the I region, since appropriate recombinants necessary to exhaustively test the serum do not exist. For these experiments, an attempt has been made to select antisera and target cells which have been extensively characterized. Experiments were also performed with several concentrations of antiserum, attempting to minimize differences in antibody titer or membrane antigen(s) density. It is possible that anti Ia sera contain undetected non cytotoxic antibodies; these should, however, be detected in blocking experiments. As previously noted, the question of nonspecific blocking via Fc receptors is often raised. The use of non appropriate targets, inappropriate anti Ia sera or anti H 2K sera as controls eliminates this criticism. The lack of inhibition with anti H 2 sera indicates that blocking is not a simple alteration of membrane function, because coupling of an antibody to the cell membrane was not sufficient to produce inhibition. Since both anti H 2 antibodies and anti Ia antibodies are largely of the IgG class, it is unlikely that the inhibition is mediated via the Fc receptor. For a difference in blocking activity to exist, one would have to assume a unique heavy chain constant region.