Synergies between the dynamics of the immune response of T cells and the variable structure control paradigm

This paper argues that strong synergies exist between the Variable Structure Control (VSC) paradigm and the dynamical behaviour of the immune response of T cells following vigorous infection. Sharp changes in T cell population kinetics in response to an infection have been revealed by experimental studies. Striking similarities are shown to exist between the phase portrait of a classical VSCS and the phase portrait of an accurate model of the T cell response with an on/off activation function. The robustness properties of the T cell response dynamics described in current experimental and mathematical studies are evaluated using Lyapanov stability theory and numerical simulations. The findings demonstrate that the T cell dynamics following vigorous infection behave as a closed-loop system under variable structure control. This control law effectively determines the immunological control structure and the metabolic energy regime required to ensure that the dynamics of the responding T cells maintain a healthy state. The VSC paradigm thus provides a mechanism to understand the transition from health to disease.