IFNG and tropical spastic paraparesis: HAM/TSP patients present a series of immunological dysfunctions, including spontaneous proliferation of HTLV-infected T CD4+ lymphocytes, an increase in the migratory capacity of circulating leukocytes, and increased production of inflammatory cytokines—particularly neurotoxic cytokines such as IFN-γ and TNF-α—in affected regions along the spinal cord [6,16,25,26,27,28].