Estimation of immune states in HIV dynamics

The human immunodeficiency virus infection, that causes acquired immune deficiency syndrome, is a dynamic process that can be modeled via differential equations. In the literature of control applications, model-based control methods to boost the immune system by means of drug scheduling have been reported. The control purpose is to steer the system to an equilibrium condition, known as long-term nonprogressor, which corresponds to an infected patient that does not develop the symptoms of acquired immune deficiency syndrome. In this paper we investigate methods to estimate the state of the immune system based on the available outputs of the HIV model. A nonlinear observer is designed and an approximate observer is also given. Computer simulations are presented to show the feasibility of the estimation methodology.