Mechanism of a transient but long-lasting immune memory function on a self/non-self boundary

Our previous study proposed an immune network model with evolving dynamics for antigen-specificity of a lymphocyte receptor, and obtained the following results: 1) the model revealed threshold dynamics to determine an immunological self/non-self boundary, and furthermore 2) the application of the threshold dynamics enabled an immune memory function. However, as the model equations were high- dimensional and nonlinear, the study could not explicate a mechanism for these immunological phenomena by analyzing the model equations. This study proposes a "minimal and analyzable" model that enables the reproduction of these two immunological phenomena. From analyses of isoclines of the model equations, 1) the threshold dynamics stems from the so-called excitable dynamics that are well-known in the firing dynamics of the neuron, and 2) the immune memory is realized not as a stable dynamical attractor, but as a long-lasting transient. This result indicates the possibility of a new type of immune memory different from the known dynamic attractor immune memory.