Figure 1.

a) Representation of the model with N cells of two cell types, APCs and T cells.All cells present ligands to the other cell type. APCs present a diverse set of peptide-MHC complexes, . T cell ligands l₁ and l₂ are less diverse and determine the T cell subtype i. All APCs of the same subtype share the same receptor. Receptors encode all information on how they interact with the other cell type ligands. This information is represented by an interaction list (IList). APCs can only have two types of receptors: either they interact with higher affinity with ligand l₁ or the inverse. T cell receptors ILists are more complex. In principle all T cells have different receptors and their associated ILists rank APC ligands p_i in completely different orders. b) When two cells interact they perform a decision that determines their next state. A new conjugate is formed whenever both cells involved receive a stronger stimulus from the new cell. In the case represented here, receptors of the T cell with ligand l₂ interact with higher affinity with ligand p_N than p₉. Consequently, the T cell tends to depolarize its previous synapse towards the new APC, in accordance with the observations in²⁸. In this case the APC receptors also promote the interaction with ligand l₂, and consequently a new conjugation is formed because both cells promote the new conjugation. In that case, cells involved in previous conjugations are freed. In this framework, it is assumed that cells always promote interactions with cells displaying ligands ranked higher in their IList. In particular, non-conjugated cells always favor forming a conjugate than to remain alone.