Human T cell lymphotropic virus type 2 (HTLV-2) is an exogenous retrovirus that establishes a lifelong chronic infection in humans and is rarely associated with disease. HTLV-2 is a common co-pathogen among patients infected with human immunodeficiency virus (HIV). HIV-1/HTLV-2 co-infected individuals have lower plasma HIV-1 levels and delayed rates of CD4⁺ T cell decline, effects attributed to Tax2, the transactivator protein of HTLV-2. Tax2 has been shown to promote survival and proliferation of T cells, and stimulate cytokine and chemokine production by peripheral blood mononuclear cells and cell subsets. Tax2 activates the canonical pathway of NF-κB, but its role in other signaling pathways has not been clearly elucidated. Herein, we review the most recent information on HTLV-2 biology, clinical features, and infection immunity using data from our research and the related published literature.