In this paper, the dynamic behavior of an immunosuppressive infection model, speci_cally AIDS, is analyzed. We show through a simple mathematical model that a sigmoidal CTL response can lead to the occurrence of transcritical bifurcation. This condition usually occurs in immunode_ciency virus infections (such as AIDS infection) in which viruses attack immune cells CD4+T. Our results imply that the dynamic interactions between the CTL immune response and HIV infection are very complex and in the CTL response, dynamics can exist the stable regions and unstable regions. At the end of the paper, numerical simulations are presented to illustrate the main results.

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