A locus mapping between the I-J and S regions of the H-2 gene complex controls the Ia.7 determinant. Studies with recombinant strain A.TFR5 reveal that although B lymphocytes from this strain express specificity Ia.7, quantitatively less antigen is found when compared with other Ia.7-positive strains. For example, quantitative absorptions with counted numbers of lymphocytes show that roughly the same number of cells obtained from Ia.7-positive strains completely absorb antibody cytotoxic for A.TFR5 target cells from a B10.A(18R) anti-B10.A(5R) serum. In contrast, approximately five times as many A.TFR5 lymphocytes are required for complete absorption. In addition, indirect immunofluorescence studies with the same Ia antiserum show that A.TL splenic B lymphocytes stain much more brightly than A.TFR5 splenic B lymphocytes.
There are two possible explanations for these observations. First, a mutation may have occurred in the A.TFR5 line that affects the quantitative expression of the Ia.7 determinant.