IFNG and tropical spastic paraparesis: The immunological response during HTLV-1 infection is complex, and although IFN-γ is crucial in fighting intracellular viral agents by blocking their replication (Schoenborn and Wilson, 2007), elevated levels of this cytokine are harmful to the host; in the case of HTLV-1, this may lead to the progression of severe diseases, including HAM/TSP.