AbstractThe structural organization of anti‐idiotypic (id) antibodies has been investigated mostly in haptenic systems. No information is available about the structural characteristics of anti‐id antibodies in major histocompatibility complex (MHC) antigenic systems, although these data may contribute to our understanding of the molecular basis of their functional role in the immune response. Therefore, we have determined the nucleotide and derived amino acid sequence of the VHand VLregions of the anti‐id monoclonal antibodies (mAb) F5‐444, F5‐830, F5‐963, F5‐1126, F5‐1336 and F5‐1419, which had been elicited with the syngeneic anti‐HLA‐DR1, 4, w14, w8, 9 mAb AC1.59. The six anti‐id mAb are heterogenous in their VHand VLregion gene usage. This structural heterogeneity is not correlated with their target specificity and with their ability to elicit anti‐HLA‐DR antibodies. The latter characteristic is markedly influenced by a limited number of amino acid substitutions, since mAb F5‐444, which induces anti‐HLA‐DR antibodies, differs only in two residues in complementarity‐determining regions and in five residues in framework regions from mAb F5‐1126, which does not induce anti‐HLA‐DR antibodies.The heterogeneity in VHand VLregion gene usage by the six anti‐id mAb in the HLA‐DR system is at variance with the restricted VHand VLregion gene usage by syngeneic anti‐id mAb in several haptenic systems. Furthermore, at variance with haptenic systems, the primary structure of the D segments of the anti‐id mAb is not correlated with their ability to induce anti‐HLA‐DR antibodies. On the other hand, the frequency of D‐D fusion events underlying the derivation of the D segments of the six anti‐id mAb in the HLA‐DR system and their average length are similar to those found in anti‐id mAb in haptenic systems. In addition, like in the latter systems, somatic mutations appear to contribute to th
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