In the absence of ARA, if an APC presents a total of 105 peptide-Class II MHC epitopes and even if as little as 10% of its total presented epitopes are self, then a response to at least 104 S-epitopes would be at risk of breaking tolerance compared to the one S-epitope expressed on >95% of the cross-reactive NS-antigens. The probability MG-132 chemical structure that an eTh anti-NS will break tolerance by signalling an iT anti-S in ARA is very low compared to what it would be in its absence. The APC would have to express <10−4 of its processed epitopes as S, before ARA becomes irrelevant to Module

2. It is possible to envisage a situation in which the APC cannot present exogeneous S-antigen by assuming that uptake is dependent on the formation of an antigen-antibody complex. This, in and of itself, would significantly reduce the proportion of S-epitopes presented. If, in addition, the uptake of an NS-antigen-antibody complex shuts off endogeneous presentation of S for a period sufficiently long for T-T interactions to occur, then activation approaching the specificity of ARA might be possible [6]. Bretscher [32–34], who has pioneered a good deal of the thinking in this field, has given us a food-for-thought

suggestion to solve the problem of ARA for T-T interactions [35]. If the B cell acted as the sole APC for T-T interactions, the fact that the B cell presents a single NS-antigen AZD1208 cell line would

ipso facto solve ARA for that antigen. The assumption that the B cell is the APC used for T-T interactions appears to solve the problem of ARA. The proposal is so seductive that one wonders why so many reasons to question it arise. 1  Mutant animals without B cells have T-responses that are normal [36–39]. In sum, this proposal is tenuous in spite of the fact that a B cell is known to be able to act as an APC. One competing assumption is that the professional APC can process an antigen into a signalling patch that maintains the derived peptides together, and across which a T-T signalling interaction occurs [6, 8]. This suggestion has its difficulties with mechanism, as does the assumption that the APC can present peptides Chlormezanone from only one antigen at any moment in time. This latter idea is an analogue of the B-cell/APC model with the advantage that it might be able to solve the problem of rare cells interacting. The almost universally popular assumption lacks rationale, namely that Signal 2 is ‘costimulation’ delivered by an APC to any iT-cell receiving Signal 1. Given that peripheral tolerance exists, a solution to the mechanism of ARA in eTh-APC-iT Signal 2 transmission is mandated [7, 35]. The postulated obligatory role for ARA in Module 3 will be analysed next. The mechanism of ARA by T cells interacting on a ‘professional’ APC (dendritic cell) will eventually have to be faced.