A new mathematical model is presented to simulate various changes of cell pools in the T cell immune system with validation procedures imitating viral infections. The present paper focuses on changes during the course of an acute progressive HIV-1 infection. Parameters are optimized by a direct search method and the stability of the model is studied. Mathematical modeling supports the hypothesis that the differentiation blockade is one major reason for the depletion of CD4 cells and the proliferation of CD38 cells in HIV-1 infection. The model appears to be a useful basis for further simulating disturbances of the T cell immune system in other viral infections as well as to elucidate the pathogenesis of various immunological diseases including the development of malignant lymphomas.