The experiments presented in this paper demonstrate the existence of T-T cell interactions in responses to azobenzenearsonate (ABA)-protein conjugates, and also make the point that the spectrum of T-cell regulation from facilitation (i.e., help) at one end to suppression at the other, which has been well documented in T-B cell interactions, is also followed in T-cell regulation of other T lymphocytes. The data extend the activity of ABA-specific suppressor cells, which were shown to specifically suppress the development of delayed hypersensitivity to ABA-T, to T cells responsible for delayed hypersensitivity to protein antigens provided immunization is carried out with ABA conjugates of these antigens. Thus, suppressor T cells acting on the development of delayed hypersensitivity are not limited in their effects to T cells bearing the same specificity but can effectively suppress responses on immunologically unrelated T cells if they are specific for carrier antigens covalently linked to the ABA-T determinant. Moreover, these studies demonstrate that, as is true of T-B cell interactions, the most efficient T-T cell interactions occur to determinants linked together on the same molecule thus supporting the concept that development of effector T-cell function may involve participation of at least two distinct precursor cells, each of which may convey independent determinant specificities and/or genetic control.

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