Gorczynski have reproduced the Takemori and Tada experiment, confirming the existence of specific T cell factors. In the symmetrical network theory tabs are able to block V regions and also to have a stimulatory role when bound to a tab receptor on A cells. Symmetrical stimulatory interactions follow from the postulate that activation of B cells, T cells and A cells involves cross-linking of receptors. The symmetrical network theory has been developed with the assistance of mathematical modeling. In order to exhibit immune memory to any combination of a large number of different pathogens, the system has a large number of stable steady states. The system is also able to switch between steady states as has been observed experimentally. For example, low or high doses of an antigen can cause the system to switch to a suppressed state for the antigen, while intermediate doses can cause the induction of immunity. I-J, the I-J paradox, and a resolution of the I-J paradox The theory accounts for the ability of T cells to have regulatory roles in both helping and suppressing immune responses. In 1976 Murphy et al. and Tada et al. independently reported a phenomenon in mice called I-J. From the perspective of the symmetrical network theory, I-J is one of the most important phenomena in immunology, while for many immunologists who are not familiar with the details of the theory, I-J "does not exist". In practice I-J is defined by anti-I-J antibodies, that are produced when mice of certain strains are immunized with tissue of certain other strains; see Murphy et al. and Tada et al., op cit. I-J was found by these authors to map to within the Major Histocompatibility Complex, but no gene could be found at the site where I-J had been mapped in numerous experiments. The absence of I-J gene(s) within the MHC at the place where I-J had been mapped became known as the "I-J paradox". This paradox resulted in regulatory T cells and tabs, which both express I-J determinants, falling out of favour, together with the symmetrical network theory, that is based on the existence of tabs. In the meantime however, it has been shown that the I-J paradox can be resolved in the context of the symmetrical network theory. The resolution of the I-J paradox involves a process of mutual selection (or "co-selection") of regulatory T cells and helper T cells, meaning that