The possible role of i region determined cell surface molecules in the regulation of immune responses.

An hypothetical model has been presented by which I region determined cell surface molecules (Ia antigens) mediate the collaboration between T cells and B cells leading to control of the humoral immune response. The model proposes an analogy between Ia antigens and the constant regions of Ig chains. The absolute requirements of this model are: a) On the B cell the Ia antigen is closely associated with the Fc receptor; b) On the T cell the Ia antigen is closely associated with the product of a linked variable region gene which functions as a specific T cell binding site; and c) The interaction between the T cell Ia molecule and its B cell counterpart leads to B cell activation. By the proposed interactive model no additional Ir gene products are required to explain current concepts of Ir gene function. The experimental evidence from our own laboratory and elsewhere upon which this model is based has been reviewed and a variety of consequences and predictions of the model have been examined. There are numerous aspects of the model which, because of a lack of hard data, are open to alternative explanations. The possible usefulness of this model should lie in its ability to suggest further experiments to elucidate the mechanism of B cell activation and control of the immune response.